I know a forum (it was related to jobs and exams in a country) which implemented something like this. It was a very popular forum as it used to share ts monthly earnings with all the members making posts on that forum. With time the number of members increased and thus they introduced some point system and posts were automatically given some points based on content length. Later members started abusing this by making irrelevant posts with more content length. That forum had to completely remove the process of sharing the monthly earnings after its search engine rankings were affected by so much spam. Thus, manual merits is best for bitcointalk.