That's what Slashdot has had for comments for as long as I remember (+funny, insightful, etc.). You can tweak your settings for how those traits are projected to a one-dimensional number (e.g. making "funny" more or less valuable).

Boy, I don't know. Probably the most important factor for a news site is interesting people commenting. I would be curious about a ranking method based somehow on how long of a thread a comment spawns. Kind of like pagerank or citations ranking in journals. If someone's comment spawns a lot of discussion (minus the spam), it is rated as valuable.

But I've concluded that I really don't understand people that well, so I don't have a good intuition as to what would actually work.