There is a golden rule of houseruling that everyone should always keep in mind when they decide to enforce a house rule, and that is:
A house rule should always either a.) make the game more fun or b.) make the game more realistic while not making it less fun.
I will draw an analogy with John Stuart Mill. A (very) rough summary of Mill's ethics: Restricting freedom by declaring a thing unacceptable (e.g., passing a law against it) is inherently an evil act; the only way for banning an act to be good is if that act is more evil than the evil of banning it. Even more roughly: freedom is good and restricting freedom is evil, so the only acts it is good to restrict are those acts which restrict freedom even more. (The garuda of Perdido Street Station are explicitly Millian in this limited sense: the only crime in garuda society is "choice theft". Anything which restricts another person's choices is choice theft, and thus criminal.)
The analogy is this: a house rule is inherently an evil. Each house rule you add makes the game slightly more confusing and gives your players one more thing they need to remember. I am lawful neutral and have a strong "rules for the sake of rules" tendency, so this is a hard thing for me to keep in mind, but it's important.
So every houserule needs to have a reason to exist. As above, it needs to make the game more fun, or it needs to make the game more realistic while not making it less fun. Because houserules inherently make the game slightly less fun, this second clause should be read as "make the game more realistic while making it also slightly more fun", unless you're playing with a table full of hardcore simulationists for whom increased realism is automatically more fun (this describes me to an extent).
That said, I've decided to make an audit of all my current house rules, to make sure they all adhere to this rule.
• Ability scores: point buy 30, starting at 8, as on page 169 of the Dungeon Master’s Guide.
• All characters begin with 1,000 gold and an adventurer's kit containing a backpack, 5 torches, flint & steel, 50' hempen rope, a waterskin, and clothing of your choice (excluding courtier's outfit, noble's outfit, and royal outfit).
• All characters begin at 0 experience.
• Content from any 3.5e rulebook or supplement published by Wizards Of The Coast may be used (including some material for other campaign settings). Other sources -- particularly Dragon magazine, the Wizards of the Coast website, and official 3.0e sourcebooks -- may be permitted, with DM review in each instance.
• A character may have up to 2 flaws and up to 1 trait (but bear in mind that you should consider no flaw “safe”; I may deliberately throw encounters at you which prey specifically on your flaws).
These are not really "house rules" so much as they are "variables that it is necessary to define for every game". I could just say "begin play according to the PHB", with the standard 3d6 ability scores and starting wealth rolled by class, but those are rules that are so rarely used they almost count as house rules in their own right. (Plus rolling for ability scores is evil and sadistic and I would never inflict that on anyone.)
Incidentally, I picked that high starting gold amount for a reason: it's enough to get anything cheaper than a +1 weapon or armor. You can afford good stuff, increasing survivability at the painful low levels, without significantly changing balance (creatures with DR/magic will still be fully effective until after you've won a few fights, because you can't afford a magic item).
• Monster class progressions (e.g., from Savage Species) may be used. A character is not required to finish their monster class progression before entering another class; however, a character may not have more class levels than monster levels unless their monster class progression is complete. The “empty levels” that do not add hit dice may be reduced like level adjustment, as below. The experience cost to do so is determined by the final LA of the monster class.
• Bloodlines (from Unearthed Arcana) may be used. A bloodline level counts as a level adjustment and nothing more; it does not count as a class level for any purpose. However, bloodline levels may be reduced like level adjustment, as below. The experience cost to do so is determined by the bloodline strength (+1 for Minor, +2 for Intermediate, +3 for Major).
These are partially laying out some specific things that I will allow and which not every DM does. So, again, variables that need to be defined for each game, rather than house rules. They also lay out some minor changes and adjustments.
The "empty levels are level adjustment" thing is simply making explicit something that was previously implicit, so not even really a house rule, per se, because it's not even really a change.
The "bloodline levels are level adjustment" are making simple something that was unnecessarily complicated. The game already has class levels and hit dice and level adjustment (which are confusing enough), and Unearthed Arcana saw fit to create a new category that nothing else uses? Pshaw. Just use one of the existing ones. So this increases fun by leading to a net decrease in complexity.
• A character may have up to two base classes without an experience penalty, or three if one of them is their racial favored class (or if their racial favored class is “any”). After that, experience penalties for multi-classing apply.
I'm starting to rethink this rule. When I instituted it, I hadn't realized that prestige classes don't count towards experience penalties. With that in mind, the default rule seems fine - you get a class plus your racial favored class, which should be plenty for anybody who isn't going some bizarre build that calls for dipping half a dozen different classes.
I do want to discourage said bizarre builds, so I don't want to just say "no experience penalties for multiclassing ever" like most DMs do. Being restricted to one or two base classes is a little harsh, though - sometimes a 1-level dip is just the thing to make a build even playable. So two or three is a fine number.
But the bookkeeping for XP penalties if anybody actually does choose to go with such a build is a nightmare, so I don't want it to ever happen. If I really want to discourage such bizarre builds, I should just say you're not allowed to play a character with more than a certain number of base classes. However, the very fact that the bookkeeping for these XP penalties is such a nightmare will automatically prevent 99% of players from even bothering with them, so the problem solves itself.
In short: not using the penalties would be going too far, and keeping them as-is isn't any fun, so I think this compromise does fall into the "makes the game more fun" zone.
• A modified version of Unearthed Arcana’s variant rule for reducing level adjustment may be used. A character’s level adjustment may be reduced at any time (including at character creation), provided the character has enough experience. You may spend as much experience as you desire, but your total experience cannot go below zero. Your class levels are never reduced in this way, no matter how much experience you spend.
Starting LA : XP Cost
1 : 6,000
2 : 11,000; 13,000
3 : 16,000; 21,000; 23,000
4 : 21,000; 29,000; 34,000; 36,000
5 : 26,000; 37,000; 45,000; 50,000; 52,000
6 : 31,000; 45,000; 56,000; 64,000; 69,000; 71,000
7 : 36,000; 53,000; 67,000; 78,000; 86,000; 91,000; 93,000
8 : 41,000; 61,000; 78,000; 92,000; 103,000; 111,000; 116,000; 118,000
9 : 46,000; 69,000; 89,000; 106,000; 120,000; 131,000; 139,000; 144,000; 146,000
For example, a character with a +3 level adjust may reduce it to +2 by spending 16,000 XP, to +1 by spending another 21,000, and to +0 by spending another 23,000.
This... I unfortunately haven't had a chance to test this system extensively. It does, however, simplify and streamline the baffling-even-once-you-understand-it Unearthed Arcana system (the UA writers must have had a fetish for needlessly complex house rules), making it probably a good house rule.
• If your name is in the "Player Name" field of a sheet, only you may play that character. If there is no name in that field, anybody may play that character. You may claim or unclaim a character at any time.
I'm actually considering doing away with this rule, and saying that pregen characters are always fair game for anybody to play. But what if a player gets really attached to one? (Unlikely, I know, what with the players not having had a hand in their creation.) So this rule is definitely a candidate for deletion.
• A character is considered dead if he reaches negative his Constitution score or negative 10, whichever is further from zero.
I play with this rule because I played with it in the first game I played in. But what's the point of it? It doesn't make the game any more realistic (high constitution already means you're less likely to die, you've already got an extra hit point for every two points of constitution you have, plus it adds to your fortitude saves.) It makes a character slightly less likely to die in one hit if they're low on HP, which is I suppose good, but is it worth making a house rule about it, given the John Stuart Mill analogy? I don't think so. Definitely a candidate for deletion.
• An attack roll of 1 is a threat for a critical miss, which works just like the inverse of a threat for a critical hit: you roll to confirm the fumble, and if your confirmation roll would miss your target, you have critically fumbled.
If you critical fumble on the last attack you would make on your turn, various bad things happen (e.g.: you hit yourself or an ally, your weapon breaks, etc). If you still have attacks left to make when you critical fumble, you lose them, but nothing else bad happens.
The grounds on which people object to critical fumbles (usually "it makes no sense for you to be more likely to hurt yourself as you level up") apply only to unadorned "you hit yourself if you roll a 1". Everything about this rule is designed to answer that objection, and it does so admirably. But does critical fumbling as a concept actually improve the game?
Yes! I refer you to the tales my players still tell of excessively powerful monsters biting themselves to death in the chaotic throes of combat. And isn't the ability to tell stories of your exploits the point of D&D? The critical fumble rule stays.
• The reincarnate spell chooses randomly from custom lists. A creature is overwhelmingly likely to be reincarnated as a creature of its type and somewhat less likely to be reincarnated as a creature of a different but similar type.
Gender is random. The original form's racial hit dice and level adjustment are removed. Half the experience points for any previously paid off level adjustment are immediately refunded to the character. If a character is reincarnated as a creature with more than one racial hit die and/or a level adjustment, the racial hit dice and level adjustment are applied to the character. If a creature doesn’t like its new form, it has the option of refusing to return, in which case the spell is wasted, as with any resurrection spell.
• The experience of dying and returning to life leaves a person drained of vitality even beyond the loss of a level. Upon resurrection, a character is aged a number of years equal to 1d20 minus their Constitution modifier (a negative Constitution modifier can increase the number of years aged). The True Resurrection spell negates this effect. These years are added to the base adult age in the case of the Reincarnate spell.
The former of these makes the game both more fun (though many people will argue that allowing Reincarnate at all makes the game less fun; these people are spoilsports) and more realistic (living constructs, native outsiders, and monstrous humanoids coming back as humanoids? engineers not having a chance to come back as engineers? Nonsense!). It stays.
The latter is an attempt to stave off the revolving door of death, which enough people complain about that it seems to be something that makes it more fun for some. That makes it good. It helps that rules for resurrection and reincarnation will only be used very rarely, so they're not something the PCs really have to keep in mind.
• A wizard may copy any arcane scroll into his spellbook, even if it is not on the Wizard class list. However, to do so he must pass an additional Use Magic Device check as if he were casting the spell from the scroll. The Archivist must do the same for spells which are not on the Cleric class list.
This isn't so much a house rule as it is an answer to a question that actually did come up. Only wizards and archivists even need to pay attention to it.
• There are no negative levels. In place of each negative level bestowed, the victim immediately ages 3d10 years. The victim may immediately roll a fortitude saving throw at the same difficulty as removing the negative level; if this saving throw succeeds, the aging is halved. If the victim receives a restoration spell within 24 hours, the aging is reversed; otherwise, it is permanent.
This makes the game more fun (nobody likes negative levels). I guess nobody can really describe the effect of having your life sucked out by an undead monster, so it doesn't really alter the "realism" scale.
• When crafting an item or casting a spell with an experience component, you may spend as much experience as you have, but your total experience cannot go below zero. Your class levels are never reduced in this way, no matter how much experience you spend.
This: probably unnecessary. I don't know that it makes the game more fun at all. I could drop it, though it would require altering the level adjustment reduction rules for consistency. Definitely a candidate for deletion.
• Participants in combat act on a shared initiative. Each member of each group (usually there are two groups: the PCs, and whatever they are fighting) rolls initiative and reports the highest initiative from each group.
In cases where there would be a surprise round, initiative is not rolled. The surprising party simply goes first, and the surprised party begins combat flat-footed.
As far as I've been able to tell so far, this house rule improves the game. It certainly speeds it up, though I have several players who don't seem to understand the principle of it at all and simply wait until everybody else has gone. I'm hoping eventually the principle of coöperation will click in everybody's minds.
• If you are a prepared caster, casting a level 0 spell does not remove it from your mind. If you are a spontaneous caster, casting a level 0 spell does not use up a spell slot.
This does not apply to Cure Minor Wounds or Repair Minor Damage, nor does it apply to any spell-like abilities. If you use a level 0 spell or spell slot to do anything other than cast that particular spell (e.g., spontaneously cast Inflict Minor Wounds by sacrificing a different spell), it still unprepares the spell/uses the spell slot. If you apply a metamagic effect to a level 0 spell, casting it unprepares the spell/uses the spell slot, even if it doesn't change the spell's level.
This: also probably unnecessary. You can accomplish much the same end by just taking reserve feats. Who actually runs out of 0-level spells, anyway? Does that happen? Are 0-level spells so useful people are in danger of running out of them?