I’ve leveled a few Warlocks in World of Warcraft now, so I think I have a pretty good idea of what works for leveling this particular class.  Though Demonology and Destruction both offer some sound benefits for leveling up, my money is on Affliction for the best warlock leveling build.  It offers the most damage at early levels, plus a ton of survivability through life draining effects.  It is also the most mana efficient talent build, because it lets you do damage with your wand rather than constantly casting.  Once you get Dark Pact, you pretty much become an unstoppable damage machine that can go for days.  As you can imagine, that makes for very good leveling.