eamon
Explorer
They'll regularly fail them though (and might not even have the skill), and the knowledge the monster checks reveal won't cover every detail.On the other hand, monsters don't really have gotcha abilities since PCs will usually be able to make knowledge checks to learn about their powers.
The idea that monsters should not have gotcha abilities is a design philosophy, which doesn't directly translate to a particular monster. Within the context of that design discussion, the type of gotcha most important to avoid are those you can't prevent - something like 3e spell resistance or miss chances, and not things that represent the unknown. A surprise can be fun, a slog through an unavoidable gotcha is better represented by simply raising hitpoints or defenses.
So surprises should happen - but not all the time. How often in your combats does it occur that a player fails to realize some in-game fact and makes a ridiculous choice? This is a far bigger problem than most metagame knowledge about which powers a creature might have, since such knowledge usually only has a subtle effect, and secondly since it's quite reasonable (in many cases) for such knowledge to be had in-game.
I've had a DM once that described a room as having a spider in the corner and calling for initiative, after which someone asks "is it big? Like, can I step on it?" The reply was "Sure, you could stand on it." In the player's mind, the spider was small, and in the DM's it was gargantuan. Hilarity ensued. Unless there's some real reason to hide powers or features, I'd generally assume that the outline of many threats is visible or well-known, and to avoid not describing it.
Let me put it this way: if the opposing party knows less than half the stat-block, then that's probably too limited a description or some unbelievably surprising combatant.