vBulletin has a feature that is sort of like that. It's sort of a "Thanks" button. What I've found is that more often than not, it just spurs on ridiculously childish behavior. Most things people discuss aren't a matter of majority rule, and so accumulating "Likes" doesn't make a point made better, yet a lot of people try to think of it in those terms. (In that way, it's a bit like people claiming post counts, or join date, matters.)