The tendency of an idea to be met with knee-jerk flames says more about the responders than about the idea's merit.

Which doesn't mean it has no correlation to the idea's merit.

Actually, I'd be curious about what makes the best flamebait -- is it totally dishonest stuff, mostly controversial stuff, or ideas that people suspect are true but want to think (or what to be heard saying) are false and deplorable?

In spite of being textual, I think most flaming really happens at an emotional level. The best flamebait pushes people's buttons (perhaps unintentionally).