Yeah one primary issue is intraparty conflict -- you need a reason for a bunch of Evil people to *want* to work together (or, you have to all want/enjoy PvP action).

Another, lesser issue is level -- a low level Evil party won't necessarily play a *lot* differently than a low level Good party, because you still have to buckle under to anyone more powerful than you (which at low level, is pretty much everyone, including town guards). At higher levels, you can start flaunting your power and doing and taking anything you want without fear of repercussion... wait, how is that different from a normal D&D campaign again? ;)

Seriously though, if you/your players want to play Evil you need to ask what you/they want to get out of the experience. What are your gameplay goals, and what are your characters' goals in being Evil? (And also... how Evil/dark is the game going to go? Are you going to descend to depravities which could get squicky for some players?)