Affliction Warlock, 29.
Its really hard to say since I've only played a handful of BGs however; here is my opinion regardless.
Like in expansions past, your primary objective/goal is to throw as many bloody DoTs on as many bloody people as possible, ontop of fear botting the crap out of healers, FCs and peeling for healers. With soul burn and seed of corruption, you can (at times) without wasting the mana and time DoTTing individual people. Generally I try to DoT 3-5 people (minions/pets included) that are within range of the Seed's blast (10 yards). Other than that, you really have two primary options. Play offensive, DoT what you can, fear bot healers, Corrupt the enemies or defend your FC and fear bot people who are on them and/or the healers, making sure stealthies have DoTs on them even if they LoS and just trying to keep your FC and Healers up as much as possible.
On a positive note; Drain Life is a great way to attack enemies before they LoS, healthstone heals (me) for 1150 (roughly) and can double if you choose the right glyph. In 1v1 situations, you should swap out your glyph for Demon Training major glyph or Healthstone major glyph so you either wipe your DoTs from the enemy when you succubus seduces them, OR you get healed for roughly 2300 hp (which is half my lock's hp before wearing balance stamina set).
All and all; I love Affliction Warlocks, rogues and druids are a bloody pain in the arse which is why I'm making a balance stamina set in case I run into rogues or druids during BGs. In arenas with the high burst damage from double DPS comps; you melt instantly even in 3's if you are peeled for you won't last as long as a FoTM or a hybrid class. So; you have your place in competitive BGs, I'm sure perhaps in 5's or 3's with the right comp you will make it work; unfortunately I haven't yet. :<