Valid points, but I think for any worthwhile comment it's easy to 'extend' the content beyond a single line. For instance, if you're asking a sincere question, you could provide some hypothetical answers, or state your reasons for asking - basically increase the level of mutual comprehension in the entire discourse.
>>Tycho+(OP)
Forgive me, I didn't mean to imply that your suggestion doesn't have merit. It does.
I just think that it's far too easy to game.
Another big issue with comment quality, of course, is in the general vagueness of it all. Jokes, for example, are almost always discouraged. Almost. On some occasions, it's quite appropriate.