DragonLancer
Hero
To carry on the trend of recent threads... 
Players who argue everything call you make, even when it makes sense to everyone else at the table.
Players who don't bother to read the rules and just coast through a game.
Players deciding what PrC they are going to take before the game has even started, and without knowing if its going to be suitable.
You spend ages putting your monster/villain/whatever together and the characters walk in and trounch him straight of the bat!
Any others?

Players who argue everything call you make, even when it makes sense to everyone else at the table.
Players who don't bother to read the rules and just coast through a game.
Players deciding what PrC they are going to take before the game has even started, and without knowing if its going to be suitable.
You spend ages putting your monster/villain/whatever together and the characters walk in and trounch him straight of the bat!
Any others?