Well right now the worst part of WoW PVP is how balances are made based mostly off of forum feedback and majority rules. Currently the restodruid and the warlocks are the bane of my PVP existence. Granted, a fire spec mage isn't ideal for say, ARENA, but I do very well in the traditional Battlegrounds, but restodruids are the new paladins. I mean as a belf mage I have 4 ways to shut a paladin down when he goes for bubble or heal. Against a resto druid with all its shapeshifting stun crap I am VERY limited in my options, and even my pumped up DPS will not make a difference in a one on one fight.
As for Warlocks, fear is still a busted piece of shit. I dont care how many times someone tells me to l2p, but being fear chained by a single lock is dumb crap. Fel Guards are fuck annoying and have created a new class of Warlock, the ScaredyLock. OMG SUMMON FELGUARD RUN AWAY!
So at this point I fight the felguard and get Dotted from behind till I die, or chase the warlock and get axed from behind. Sure, I could use blink to catch up to the lock, but then i'll just be feared. Sure I could sheep the lock, but he'll probably just trinket out, or deathcoil me during the cast. Plus he gets healed during MY cc, unlike fear which makes me run around and take damage. Fear is guaranteed TIME, while a SCORCH can break sheep. BULLLSHIIIIITTTTT
Other than that WSG still has a stupid exploit on alliance side. People have figured out how to get on the ledge BEHIND the flag where mages cant get line of sight even if they have range and no one on horde side has had the time to figure out how to get back there. So during flag standoffs you have jackoff druids with the flag hiding up there immune while everyone attacks the horde fc WTF WTF x2.
Other than that I do fine against most other classes and yes once in a while I kill me a high level restodruid or warlock, but generally as a Mage i've learned to pal around with a pair of Taurens and choose targets of opportunity, which is what a shadowpriest should be doing too..
Unless youre a holy priest, the class that will always die in almost every BG encounter. Paladins/Druids/Shamans make much better PVP healers.