Of course that invites much spam but I see it as a game: Find the good and reward it, ignore the bad and punish the ugly. I think it works ok but it takes time for each user to get it.

I like this strategy. It’s a good way to try and filter out the useless spam. It’s sad when someone posts something, expecting to get actual feedback (good or bad) and they get a fake comment instead, which is usually in the form of a compliment.