What makes an Anti-hero likable?

They're anything from merely obnoxious to straight up evil...but they're fighting other, badder guys, so that means we have to root for them, right?



What exactly makes anti-heroes of any stripe likable? What lines must you walk in order to make them work? I'm a fan of several myself (Alucard from Hellsing, Kratos from God of War, and so forth), but what makes you sympathize with, or at least root for one?



And where do other anti-heroes fail at either criteria? As someone who'd like to write an anti-heroic protagonist some day, I'd like some pointers. :P