Yahoo Answers is pretty interesting and it works on a point based system. Basically, the more questions you answer, the more points you gain. You also get points for "voting" on the best answer to each question (not as many as answering though). If the most people vote for your answer as "best answer" then you get 10 points.
That may have been a confusing explanation, but I definitely like the system (which keeps users engaged). And Hacker News does this as well with its karma system, although it works in a different way. Some of my friends are hooked on to this one chatting website (www.zobe.com), and it has a karma system as well. Basically, all the sites i've seen that have a point based/karma system are quite "vicious" as you put it (they have very consistent users).
I also forgot to mention- I also began recently using Quora, and i've liked it so far! Reminds me of Yahoo Answers but it definitely seems newer and fancier.
I'm interested in hearing more about the project you're working on! Please elaborate on what you're trying to do :)