Here's an intriguing concept: an "inference engine" that claims to rate the aesthetic values of photographs!
Obviously it's more a talking point than anything else (and, maybe less obviously, "aesthetics" isn't always the most important quality of a good photograph.) But it seems to do an eerily good job of recognizing the kinds of photos that get lots of stars of Flickr-type ratings sites.
So far I've only played with pointing a couple of my own images at it, and I'm baffled by what makes it go. For example, it liked this image a lot, giving it an 87 out of a possible 100:
And it liked this, giving it 82 out of 100:
But it didn't like this at all, giving it only 22.9 out of 100:
And my big winner so far, getting 99.3 out of 100 (!!) is this:
Now honestly, does that make any sense to you? It seems to like grainy b&w photos of cars, but beyond that...?