Not a bad idea. Popup on thumbs up/ thumbs down. Particularly useful for down votes, where an unexplained karma down vote often just seems impolite to me - it's just not very constructive to say "you stink" if it's unclear what you're complaining about.

But I maintain that the main problem here isn't that we have too few ways to vote, but too few ways to use votes and other information to filter posts in a user customized fashion. Well, I shouldn't say "we", because some people have a further problem that they just don't want some posts to exist, or be seen by anyone.