The current challenge is my first and I have to say I think the anonymity aspect is fun and challenging! It's fun to try to match the other writers with their fics, it's fun to get unbiased feedback, yet it is really hard to not respond to that feedback! I think it is too big a part of the whole experience to take away that aspect of the challenge, though.
I really loved reading all the challenges that involved another element, such as, Challenge in Two Parts, Small Things Made Large, and the Icon Challenges. And when I was reading them, I thought about how much fun it must have been to write those fics with the additional challenge of tying your fic to some other body of work. So these are the types of challenges that intrigue me the most.