One thing as a DM that I used to hate, both in 3e and in 4e, was whenever there was a fight, it always got interrupted by at least one player wanting to make a knowledge check about the monsters.
That lead to everyone in the party making a knowledge check, because it's a free action, hey why not?
So then if there was two different types in the fight, say a fey and an undead... different people are making different knowledge checks, all to try to find out resistances, vulnerabilities, creature type, hit dice, what type of creature it is, etc etc...
When all is said and done, these knowledge checks took up game time, led to metagame thinking "hrm I rolled this much and missed the DC so this creature must have at least X hit dice..." or "hrm I rolled a 32 on my knowledge Arcana check and I have nfi what this guy is, he's probably not an outsider... hey joe roll a knowledge religion check because maybe he's undead..."