Seems like a reasonable goal to me. I already know that a number of people use POPFile to do things like sort "interesting" from "uninteresting" messages (e.g. one journalist uses POPFile to automatically filter out press releases that don't interest him).
However, one issue with comments is that they can be short leaving not a lot of text to work with.
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.
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.
However, one issue with comments is that they can be short leaving not a lot of text to work with.
John.