A system that has a means of punishing the offenders or disobedience, should have a means of rewarding the law abiding members. When there are no means of rewarding goodness, it will surely decline.
For instance, if there is a reward for a newbie who under few months grew an account to full member without spamming or plagerising, it will in no doubt jealous others to begin to make merit-worth posts.
Can you give an example of what kind of rewards system you are talking about? How it would work, and where do the rewards come from. If a newbie manages to rank up to full member (and some have even ranked up to legendary), the whole community took part in his progress. That's the reward. The 100, 250, 500, or 1000 merits he received along the way is the community's way of saying we see you and we appreciate what you are doing.