I really like this idea. Or maybe a thumbs up/thumbs down system?
My only concern is the fact that concepts can be edited to the point that they no longer resemble what they once were, so the "likes" on a particular concept might reflect how much some previous version was liked. We could make the statistics reset on edit, but that wouldn't be fair for small changes.
Edit: In response to the point about the forum being sufficient to give feedback, I usually only post if there's something particular I want to point out ("this part isn't implementable", "under the following circumstances it could be overpowered", etc.), or if I really, really like it. Otherwise, I'm not inclined to waste space with a post like "I like this", or "I don't like this". Looking at the concepts forum, most of the posts are about how people think the concepts should be modified, not about whether they like them or not.