I founded and am the sole moderator of r/apachespark. That one's easy because it's a well-behaving crowd and the purpose of the subreddit is obvious. The only real things I do is remove spam/unrelated material and tag big releases as announcements.

More generally, I think the moderators' main job is to set the mission/rules of the subreddit, and enforce them. Subreddits with weak enforcement rarely thrive as they devolve into the least common denominator.