I think the most controversial idea was vampire bordello in Goodbye, Iowa. No matter how dirty and disgusting writers made it look, the very fact of humans and demons could exploit each other was a bomb.
I agree. This was one of the ideas that finally overthrew the concept of moral superiority of the soul having.
I was quite ok with it, since I was never really comfortable with the concept of the soul as a conscience. After all humans commit atrocities too. I liked it that souled Spike was so very different from Angel. It made the soul concept even more diffuse but much more fun to work with.