Like button would be a cool addition. Should have a dislike button too. You see it a lot in comments sections, youtube, etc. They use it on a baseball forum I lurk at and it's kinda cool.
And it does differ from karma points cause you can only give someone karma so many times.
Ideally someone's green-bar rep would be partially determined/calculated by likes vs dislikes on their posts.
Not that it's a huge deal, but as far as feature ideas it'd be a quick win. I'm sure it's already available as a plug-in.