It is going to be hard to make an algorithm which can't be beaten (e.g.) by just applying more adjectives and synonyms. Which can easily be automated.
Actually, I strongly doubt it can be easily automated in a way that doesn't reduce readability, which I expect to lead to a sharp drop off in number of up-votes. If the number of up-votes drops off faster than the advantage gained from the longer post, the system can't be gamed.
I don't suppose this is already being done, is it?