I have a challenge DCC, but I keep pretty good watch over it. It's not insanely active, it's pretty moderate. It's easy to keep people on track, but they rarely astray from discussing challenges. And I've always wondered that too. I know Hiidoran IP checks every new member, so that's probably one of the hardest mod jobs imo.
Something I'm about to finish at this very moment is the triospecies challenge. I think it's posted for all games, but you choose three Pokemon (obviously) and go through the game with them. It's pretty easy and I've really enjoyed it. I'm doing it on Ruby with Blaziken, Flygon, and Glalie. :D