I think it would be a good idea to have an extra criterion for usergroup promotions under Content and Achievements called "User has started at least X threads". I could be wrong, but I believe starting new threads is more valuable in terms of search engine ranking than posting in existing threads. Also, it takes more effort to think of a new topic and start a new thread than to simply post in an old thread. I want to reward those members who make the effort. This would hopefully also be easy to implement, so it's a low-hanging fruit.