If Bitcoin Talk introduced a shop, and one of the items for "sale" was the right to display a signature, then signatures could become a privilege that had to be earned through responsible posting. Credits could be removed for bad posting, and added automatically for responsible posts.
It can work, but can be abused as well like all other systems around.
A good/responsible post could be a post that earns Merit(s) which would automatically earn credits for the member. And a bad post could be a post that has been accurately reported and deleted by a mod-- resulting a deduction of credits. Earned credits should be given per Merit received by the post-- e.g: 5 Credits per Merit or something. The same should apply for the deduction. If a user earns 2 Merits and gets one of his posts deleted in a thread, he would still have credits for 1 Merit.