I would think short comments are a stupid-positive flag all by themselves, though. It's very rare for few words to carry great meaning online; in this case, a manual "this is wise" button may be appropriate.
If I used that heuristic on email, most of PG's mail would be in my spam folder.
I agree, although there's nothing wrong with using 'comment length' as an input to some learning system. In POPFile we use quite a lot of 'meta' information about email including length to make a decision.
John.
did you keep your comment short to make your point?