High damage + Soul Leach absorbs. Look at heroic Tortos, for example: A warlock, properly played, never needs to use the encounter-specific shield on the fight. Soul Leach, alone, is enough to completely absorb the damage from Quake Stomp.
Then there's sacrificial pact, which is extremely useful on nearly every heroic fight. Ionization on heroic Jin'rohk? Sacpact! Dire Call? Sacpact! Quake Stomp? Sacpact! Quills? Sacpact! Wind Storm? Sacpact! Static Shock? Sacpact!
We also are extremely mobile: We can cast while moving, we can teleport, and we can provide gateways.
Now, don't get me wrong... I absolutely LOVE everything I just said. You have to admit, though, that our damage plus our survivability is a bit OP. A decent chunk of this is getting fixed in 5.4, so we will no longer be OP.
Now, when it comes to PvP, I have nothing to say. I don't PvP and I've never felt particularly strong when I have PvP'd in this expansion.