Just an idea (and probably a poorly thought out idea), but perhaps there could be something that shows the ratio between number of threads started and number of threads answered in. My theory being that if you started a large number of threads you are probably asking a lot of questions, and if you just contribute to pre-existing threads you are answering questions and, therefore, helping lots of people.
Sort of how much you take vs. how much you give.
I dunno if introducing a scoring system that might strongly influence people's posting styles is a good idea, but I thought I'd just put it out there. What do you think??