I agree with Simon's point and also with Danee's. I also agree with AElf's point that it shouldn't be seen as some sort of competition. I think that you might consider something along the line of degrees of enlightenment. How many degrees I don't know. There, that should provide some fine lines to walk on.
I don't wish to unnecessarily complicate things but can you bifurcate the box score somehow so that both number of posts and number of topics started are considered?
How about a scale that goes from Champignon up to Champion, i.e. interested but sort of in the dark up to well-informed contributor?