I think that having this would discourage conversation. When I vote I usually do so for the posting with the main idea and not for the subsequent postings from the same author.

I do think this would be useful but not in such a crude manner as dividing votes by number of posts. Maybe we could take the sum of votes per discussion thread to elaborate this index. This wouldn't discourage conversation as much and would still be meaningful.

$MonkRating = $SumOfAllVotes / $NumberOfThreadsHeHasParticipatedOn;
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Still I don't feel too comfortable with this idea. It may be fun but it will probably intimidate newcomers.