The Warlock is the attrition class. You do not burst things down, you grind them out. Curses and damage over time eat away at your target while your Voidwalker holds it, Drain Life keeps you topped up, and by the time the mob realizes it is in trouble it is already dead. It is slower than a Mage and far safer, because a Warlock with a tank pet and Drain Life is almost impossible to kill while leveling.
At 60 the Warlock is a raid pillar for two reasons. First, the damage is real, with the SM/Ruin Shadow Bolt build putting up strong sustained numbers. Second, the utility is unmatched: Healthstones for every member of the raid, soulstones to battle res a healer, summoning to drag in the four people who are still flying to the instance, Curse of Elements and Curse of Recklessness to boost the raid's damage, and Banish to lock down a demon or elemental.
The cost is soul shards. Everything good a Warlock does (Healthstones, Soulstones, summons, the pet summons, Shadowburn) eats a shard, and you farm them by Drain Souling things. Managing a bag full of shards is part of the lifestyle, and a Warlock who runs out at the wrong moment is a sad sight.