Ah ha! You've hit on my true anti-kink. No matter who writes it or how well-written it is, I cannot stand human Spike. Uh uh, no way, no how. I can recognize that it's well-written and respect the author's talent, but it's a complete and utter squick almost to the point of making me physically ill.

It's hard to explain but it sort of ties into the anti-slavery kink as well. Part of the Spike I love is the demon, and if you take away the demon it's like giving him a lobotomy, then forcing him to live on as something utterly changed, doing things he'd never have done before. And having power over the self taken away (through drugs, brain damage, mental breakdowns, happiness in slavery, etc.) is something that I've always been terrified of and horrified by more than anything else.