Originally posted by EvilVampire
In the case of Willow, it's the other way around, I think. As far as power is concerned, IMO she clearly had (has?) enough of it to be considered a "Big Bad". However, I don't think she was evil enough to be considered one. True, without Xander's intervention she'd have killed everybody (or not?), but ultimately, she stopped because of the humanity in her (that is, because she wasn't totally evil). I can't imagine any of the previous "Big Bad" stopping for that sort of reason. They stopped only because they were defeated by a stronger opponent (or opponents), but if they (any of them) had been powerful enough to defeat anybody who got in the way (as Willow was: according to Giles no supernatural or magical force could stop her), they wouldn't have stopped. Willow did, and that's one of the reasons why I think she doesn't belong in the "Big Bad" category. Also, I'm not sure she had full control over her actions. Plus, the reason why she wanted to destroy everything was entirely different from the other villains'; not that the motive makes her not evil, but I think she was "evil" in a different way (and not totally so).