1) The trading system is now broken: You can easily trade a hard-won (although useless to YOU) item for a dupe that can poof on you any second. You play the game, earn a rare item, trade it for an equally rare item (say, an Ohm Rune for a Vex Rune) and it poofs!
2) Hardcore "legit" players can get PKed
3) Often quests will be ruined for players by MFers in Public Questing Games (I've done this myself a few times)
Welp! Dinnertime! I'll check back later for more complaints... during a lull in writing my lab report this gets (sadly) quite entertaining.