[zacharycohn]

It's all about establishing ground rules. /r/askscience is the posterchild of this - they have 250k+ subscribers, and the discussion is still interesting, thought provoking, and on topic. Other subreddits, like /r/gaming, have devolved into nothing more than image macros and meme posts. The difference is 100% the ground rules that are set, and then the moderators sticking to those ground rules.

If I were in control of Reddit for a day, I'd create a "How to Build Community" packet for to give to new moderators. Often they're just promoted for being good members of the community - but that doesn't mean they know how to run or grow a good one.