Another issue that pvp general does to MMOs is that it tends to adversely affect the pve aspect of a game. An ability with an effect that is considerably powerful in a pvp environment would end up getting adjusted so that it isn't quite as powerful. However, it results in that ability in becoming lackluster, if not useless, in pve environments (especially raiding). This is my biggest gripe about games with pvp (especially world pvp).

Guild Wars 1 (and I'm hoping Guild Wars 2) eventually started getting it right with giving each ability two effects: one for pve and one for pvp. So they can tweak one aspect without adversely affecting the other.