RuneScape. Love the game, hate the community. Currently taking a break.
Spiral Knights, I love the game bordering on obsession.
The only thing I hate about the game is the people who hold up teams because they start moaning, bitching and raging about players who "steal" loot monster drops;
One of you stole MY loot/Health Capsule from MY kill. I hate thieves and whoever doesn't own up gets kicked!
...for 10 minutes. Bare in mind that cash and heat is all shared between everyone in a team, and Health Capsules and Status Vials are free-for-all, and Materials are awarded randomly to one user in the team.