Alright, I have to mention this, my favorite trap that I've never had a chance to use.
The PCs enter a room with a gleaming black floor, it looks like it could possible be stone of some sort. There's some sort of condition to disarm the trap, either a panel, or a passphrase, depending on the needs of the setup. Detect magic notes some sort of evocation effect covering the floor, but poking and prodding doesn't really seem to do anything. As the party crosses the room, the wall of force that was pressing the black pudding flat vanishes, dropping everyone smack into the ravenous ooze monster. Heh, pudding.
Just curious about everyone's thoughts on what makes a mechanic work or not.
What makes me crazy are disassociated mechanics, those being the ones that don't make any sense from a character's perspective as opposed to the player's. Things like per day limits often fall into this category. Does a cleric have any explanation why his ability to channel the divine power has seemingly arbitrary limits to how many times he can do it per day? Or how does a barbarian describe his capacity to tap into the power of his inner rage, and how that limit at might seems like an absurdly brief time when looked at from outside the frame of reference of combat rounds? We know why these are the way they are for purposes of things like "balance", but how do our characters make sense of them?
"Thog can only be angry for 36 seconds?"
If anyone is curious, here's an excellent article on the subject I came across a while ago.