When a player asks, "What do I see?", the average DM finds this a perfectly reasonable question, and attempts to answer the question (unless it's dark, or the relevant character is blinded, or whatever). Same with "What do I hear?" These are your standard-issue primary human senses, how we get most of our information about the world, and thus naturally how the DM conveys most of the information about the characters' world to the players. The DM might reasonably call for a Search/Spot/Listen/Perception check, if there's information that would be available only to particularly observant characters.
When a player asks, "What do I smell?", "How does it taste?", or "How does it feel?", the DM might be a little taken aback, as these are unusual questions, but will generally still attempt to answer the question. Again, perhaps with a Perception check, though one might be at a bit of a loss for what skill to use in 3.5 -- probably a basic Wisdom check. It's been said before (probably by me, among many others) that a decent DM will volunteer sight and sound data, while a masterful DM will neglect no senses when giving information. (This can be overdone -- nobody wants to hear a full discourse about how the air feels and tastes on
every room their characters enter -- but underdoing it is a much bigger risk in practice.)
I leave as an exercise to the reader coming up with situations in which lesser-known senses, such as
proprioception, might come up in play.
But when a player says "I cast
detect magic", the common DM reaction is frustration or annoyance. Especially in Pathfinder, where cantrips/orisons like
detect magic are at-will, so players have been known to "spam" them. The ubiquity of
detect magic is, in fact, quite a common complaint I hear about Pathfinder. (To a lesser extent, the same goes for paladins and
detect evil, even in 3.5.)
I've seen more than one DM frequently resort to "The area in general is so powerfully magical it gives you a headache and you can't pick out individual magic effects." This is may be a legit technique for lending particularly epic or eldritch locations an extra element of "you're dealing with things you can't understand", but, if you use it, you should definitely reserve it for, say, the final dungeon of the campaign, where you're walking into the lair of a physical god. (But even then, if you're high enough level where you're a reasonable challenge to said physical god, one would think your techniques for casting
detect magic would have advanced along with everything else on your sheet, so even in the presence of supremely powerful magic, you can still pick out details.) Fun fact: There are, in fact, rules for detecting "Overwhelming" auras with
detect magic, but they're limited to epic-level spells and artifacts.
And if you do that, you should do it for other senses, too. A mission to the elemental plane of light, where everything is so bright that it gives you a headache even if you keep your eyes closed and there's no way to pick out visual details. Or the plane of darkness, where no light sources function. A cacophonous factory floor, where you can't discern individual sounds. Perhaps even your sense of proprioception fails when you come in contact with stuff of the Far Realm. The one such example that I have seen DMs use fairly often in practice: stenches so strong they make you gag.
As with most DM sins, I've been (partially) guilty of this one myself. I have been known to say "This entire area is permeated with an ambient aura of evil", in the case of an area proximate to a permanent portal to the Abyss, when paladins were using
detect evil. But! This is a case of
more information, not less: the ambient evil didn't render them unable to pick out finer details of evil. When there was a demon in the next room, they could still pick out the demon's evil aura from the dungeon's ambient evil aura. And meanwhile, they had the information that there's something so evil that's been here for so long that the evil has soaked into and permeated the very stones themselves.
Anyhow: why is the DM reaction to "What do I see?" so different from the DM reaction to "I cast
detect magic"? They're both usage of the character's defined abilities to gather information about the world. Perhaps it's that the rules for
detect magic are slightly complicated and the DM doesn't feel like dealing with them. Perhaps it's that
detect magic, unlike sight/taste/proprioception/etc, is not a standard-issue primary human sense in the real world, so it seems more legit to deny characters the full use of
detect magic than it would to deny them the full use of their other senses.
As I alluded to two paragraphs ago, it's about information. As I see it, one of the DM's primary roles is to provide information to the party. (There's also some business with deciding what information is available, and under what circumstances, and of course deciding what the information consists of in the first place, but the primary business, for the purpose of this post anyway, is providing the information to the party.) Any situation where the party requests specific exposition (that their characters would reasonably have access to) is an opportunity for the DM to maximally fulfil this role.
So whenever a player asks for information, it warms my withered black heart, because it's an opportunity to provide exposition (and, often, to come up with information on the fly, which I enjoy and am decent at). Some DMs will try to restrict the flow of information, either because they think it increases the challenge or because they're just not good at or don't enjoy coming up with new information on the fly. I don't have much to say about the second thing, but I'll digress for a paragraph on the first:
Security through obscurity can make for an interesting fight,
if used sparingly. Trial and error can make for an interesting battle. The minionry of
Dr. Blelyj once fought a monster that, unbeknownst to us, was made more powerful when it was subjected to
magic missile. Many lulz were had at the expense of the poor sorcerer who'd thought his
magic missile was a sure thing. There are many clever monsters of this sort: albino red dragons, those gas spore monsters that look like beholders, mimics (and the general venerable and well-populated genre to which they belong, "monsters that look like harmless objects"), shambling mounds ("oh, it's a tree monster, trees usually get wrecked by lightning, right?"), and so on. Withholding one key piece of information can be a fun lark. (If you do this much, you should also make sure that your monsters don't always act as though they have complete information about the party.) But many players (myself included) don't care for every battle to be trial and error. Usually we just want to deploy our tactical abilities against the monsters' tactical abilities, and prefer to have more complete information rather than less (and sometimes invest substantial character options into perception and knowledge abilities, and it's generally not a good idea to deny players the fruits of their character building without a good reason).
Which brings me to knowledge skills! There are rules for knowledge skills, which you would do well not to ignore. The rule of thumb is "one piece of information for a DC of 10 + the monster's HD, and an additional piece of information every 5 thereafter". You don't need to indulge player requests for specific bits of information ("What kind of DR does it have?", or even "Any major weaknesses?"), but it never hurts.
My main beef with the knowledge system (in fact, one of my beefs with d20 in general) is that knowledge skills can't be used untrained. My beef with this is threefold: one, it restricts me as a DM from being able to give out information, and I always love giving out information; two, it restricts me as a player from acquiring information, and I usually love acquiring information; three, it forces me as a DM into a position where I have to say "no",
which is Bad. I keep thinking of instituting a "you can use trained-only skills untrained at a -10 penalty" houserule. Other DMs I've played with have dealt with this by expanding (usually on the fly) the list of relevant knowledge skills for a piece of information: to identify a given undead creature, you might be allowed to use Knowledge(dungeoneering) or Knowledge(arcana) if you don't have Knowledge(religion), for example.
Anyway, I generally encourage players to invest points in and use the Knowledge skills, because I enjoy providing exposition and it's a good tool for that. And yes, like everything, it can be overused -- I play with a druid who tends to ask if she can use Knowledge(nature) to identify everything she encounters, no matter how obviously non-natural. But, in general, allowing players to roll Knowledge checks is generally better than not allowing it.
Do note that "Knowledge" isn't just book-larnin'. It isn't always "I read about this in a book once" (though for some characters it might be). It's a combination of that, practical knowledge ("I encountered something like this once"), observation and extrapolation ("Look, it's got flattened teeth, it's probably an herbivore, though that doesn't mean it's necessarily harmless"). So think twice before using the "Nobody has seen one of these for centuries, you can't make a knowledge check" line. (Nobody's seen
the Others for centuries, but people still figured out they have DR/dragonglass or Valyrian steel, through practical observation and also it was in at least one ancient book.) Actually, that's a general principle: a thing being difficult means you should pile circumstance modifiers on to increase the difficulty, not "you can't do it". If you can
swim up a waterfall with a high enough Swim check, you can deduce the properties of an ancient monster with a high enough Knowledge check. "Don't even bother to roll" is if it's too easy to fail, not if it's too hard to succeed.
