Myself I voted for Rogues , being clearly the most overpowered class with Warlocks and Beast Mastery Hunters , and I know somebody has to comment that but I'll ignore it.
Rogues just needs a rather amount of skill to completely rape everybody in Arena/BG. They will be surely "nerfed" by the next patch where Blizzard is gonna balance Cheat Death so no more PvE Equipment in PvP enviorment . That's one of the few things that Blizzard did that is good, though that will nerf Rogue levelling quite much. Actually at this moment PvE equipped rogues should beat PvP equipped rogues with ease.
Warlocks have been nerfed in almost every patch and yet , one of the best classes and rather easy to play with the SL/SL combination. Only thing you need here is to spam dots and fear with a perfect timing so you can Drain the crap out of your enemy.
BM Hunters - What can I say here ? Their pets can smash your face within seconds with their monster damage. I think this is one of the few classes that can kill "skilled" rogues due to numerous kiting spells.
Also , I have a mage on retail and for those who voted Mage cause they killed a few noobish players they fail. I once killed 2 frost mages at the same time and it was freaking hard , I didn't use any AoE spell for those who think "omg mage aoe kkezthx". AoE spells have a global cooldown , I'm speaking here of arcane explosion mostly since it's the most decent AoE spell mages have so even like that it's rather hard to own 2,3,x people with that. All I can say to you is this : You fail , go learn some more theorycraft , as a big noob I am I know a lot more than you.
Edit - Spelling errors.