I know a lot of people have mixed feelings about Guild Wars, but I really like it and literally I can login for just five minuets, get something done and then leave the game anytime I want. Just click logout and it is done.
This is very significant for an MMO when you consider that a lot of them like Matrix Online force you to stay in the game longer than what you probably want to, because to exit Matrix Online you first have to find an ingame phonebox and then select logout. To get to the phonebox you will probably get stuck in combat or something, but this is annoying if you want to quickly shutdown the game because of a phone call, someone at the door, something on TV, dinner is ready or find that you suddenly have to leave home. This forced me to Ctrl+Alt+Del the game a few times and it is irritating.