PvP OR PvE... Of course PvP. PvE is only fun because you do it with your mates, and it can be relaxing. PvP is fun because you do it with your mates and it's tense and challanging.
Now if you're REAAAAALLY serious about making a game, it'd be sweet to make a game where users could create their own content. ATITD (A tale in the desert) is a good example of this, players making content for eachothers while the devs works on getting the next patch out, kinda.
Apart from that, Roleplaying is the part I like the best. For instance, WoW:
MMOLG
Massive Multiplayer Online Levelling Game.
Roleplay my ass. To most people nowadays, roleplays = levelling, which to me = wasting your frickin life.
OOOOOOH; I*M LEVEL 75 NOW; I*M SO MUCH BETTER THAN 74 BECAUSE THIS GAME TELLS ME SO! NOW MY BODY IS FEEDING ME **REWARD DRUGS** SO I CAN FEEL LIKE A GOOD BOY!!!!
Notice that I'm still on strike from the shoutbox.
EDIT: Also, I feel that a quote is appropriate
Sparrisen... That was** AMAZING**!!!