I like the Reputation and Hero points system, used here. It seems like it really helps increase user engagement.
I am wondering if WPMU DEV has a plugin, to allow us to do this on our own (or clients') websites. I scanned through the list of community plugins and didn't see anything, which looked like it included this.
If not, please develop this! I create local community directory/guide sites, and would love to be able to add this functionality to them.