I do not know what other people think though, but personally, I do believe that number of positive karma is or are supposed to play the major role in user's ranking, alongside the activity score, just same way activity score and number of earned merit is what determines user's ranking on btt, and of course, teleporting users from btt can always continue here with their rank on btt, this is to encourage more experienced users from other forum to join here as well.
When number of earned postive karma, alongside activity score is what determines the rank of a user, this I believe will contribute greatly to the reduction of spam and useless posts, users will be more serious to post useless things or stuffs that will earn them positive karma, which is what they need to rank up.
So, I think op's idea is a good one.