I spend a lot of time on Metafilter now. Metafiltter has a favourites system - basically a like button (no dislike button). I find it works quite well, though not everyone even agrees what it should be used for. (Some people use it to bookmark great comments, some people use it to show they agree with a post, some people use it to mark a post that they want to come back to, to argue with, some use it to offer another poster moral support).
Every so often I look around BL and see a post that I want to favourite, and then forget I can't. I can see the value - I don't always want to type a reply, it can be better just to endorse someone else's. But I'm not convinced it's right for BL. Favourites are only as good as the people giving favourites, and even on MeFi (a very smart community) they can be handed out badly (e.g. a one-line snark against bankers will get 60 favourites, a well-written argument in favour of bankers will get none).
Favourites are slightly different from reputation. Favourites would accrue to a post, reputation to a poster. Reputation was tried once in the past and didn't work, for reasons given above. And also, there's nothing stopping people from giving all the mods reputations of 1, as a joke, and giving trolls reputations of 5.
So: I like the idea in some communities, not convinced it's a great idea here, even if it's possible.