I can agree with a comment and not want to karma-reward it, if it's mundane or repetitive. That might even go so far as to agree with the sentiment but find the comment of punishable quality, a sort of "OK but enough already".

The idea of weighing ratings from commenters in the same thread more highly is interesting.

Discouraging votes-without-reading and allowing quick corrections, as with comment text, both make sense to me too.