If you're on the bloodlust battlegroup:

Addons:
Gearscore (get it so you know what your score is)
ElitestGroup (tells you if people have shit gear / enchants / gems / pvp gear /etc)
Bimbo - Reminds you to gem / enchant your gear
Rankwatch - If you're using the wrong rank spell because you (or a bug) screwed up your bar? Prepare to get kicked from groups quickly if you have this happen to you.
DeadlyBossMods - you should have this installed, but if you don't, you're doomed.