Looking at alt character builds...

[Rinnhart]

This isn't really true. Affliction builds tend to have almost no downtime, especially after Dark Pact, which makes up for the generally slower killing speed. Also, once you get high end gear, your three DoTs will do ridiculous amounts of damage. In particular, an affliction lock (especially 30/21 affliction/demo) can pull/kill an entire satyr camp in Felwood at once by just throwing Corruption/Agony/Siphon on everything while running around and using Lifetap to get mana back.

Arnath
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Note the "Spice it up with half-zone pulls."

Warlock killing speed, for individual targets, is slow. Damage over time effects require time to do their damage. As I said, they're efficient, but the lack of downtime doesn't make up for the pace of their damage, unless you start shadowbolting everything (which kills your efficiency). Closer to 60, yeah, warlocks pick up the pace, but until you're 40 with Dark Pact and and then 45 with the respec to demonology/affliction, you are neither efficient nor fast. In fact, until 40, my warlock rates as the most frustrating, easily the worst leveling class I've played.

They don't have the staying power of paladins, priests, or druids, the raw damage of mages, warriors or rogues, or the aggro control of hunters ( in conjunction with their pet). They have fear, which while powerful, is still limited.