If you reply more to a thread with quality posts, in theory, you are more "active."
How would it be able to check the quality of your posts? I kinda like the idea tho.
I am not sure how hard it would be to implement, but I like the Reddit system where people are able to up-vote or down-vote individual posts. People can then be rated on the quality of their posts (i.e. karma) by calculating the difference between their total number of up-votes and total number of down-votes. Another good thing with this system is that you can set an auto-hide function for posts which reach a certain number of down-votes (i.e. hides bad, useless or spam posts).