I once watched a livejournal group die in < 24 hours because the mod suddenly decided it was too dead and put in a rule saying anyone who didn't keep up a certain level of posting would be evicted. the thing is, it really didn't matter if no one posted much; posts were aggregated with all the other groups and journals people followed, so when someone did post something it was seen and the large lulls between posts didn't bother anyone. the minute the new rule was announced there was a general stampede for the door and the mod left there saying "but I was only trying to revive the group!"

my general takeaway from that was that people might get annoyed when you tell them what they are not allowed to do on your site, but most of them will roll their eyes and put up with it. if you tell them what they have to do they will simply drop the site and never look back.