I've noticed that once people become moderators on a site, they appear to lose interest in participation, and seem to think their only reason to be onsite is to censor other people's contributions. Rightly or wrongly. This appears to be the case regardless of whether they remain a moderator, or not.
Please note that this is <b>not</b> aimed at anyone in particular, <b>nor</b> is it aimed at all moderators. This is just a general observation, based on limited experience.
Given that moderators are usually selected based on their contributions, the idea that making a user a moderator will stop their contributing is a hard one. I think this is not good. Perhaps moderator groups by their nature are counter-productive to an lively and interactive site?