Something which most oldish Internet users may relate to is the observation that communities turn "bad" over time. There's regular discussion about it on almost every forum I've come across - be it HackerNews, reddit, or any number of public forums & mailing lists - that the signal to noise ratio is falling as new users come on board.
Absolute growth in user numbers is certainly part of the reason but I wonder whether there is an observational bias in the form of personal growth that is perhaps overlooked.
Presumably when you join a group its because you have some interest in its main topics - whether it be knitting, or rocket surgery. Initially you're new to whatever it is and every post is interesting when you know nothing. Over time you build your own knowledge and you start to see reposts, perhaps a few inane seeming questions (why didn't they do any research of their own?), and recognise FAQs.
Given that perspective, is the community's collective value falling due to the group growing in size, or could it be that you as an individual have grown beyond the group?
A simple example is the common complaint about reposts on link sharing sites. To those long tenure members who have seen every post since the beginning of time, reposts and rehashed discussion from new members are extremely frustrating (at least judging by the complaints). Is the issue though simply a mismatch in experience?
If you recognise that you've grown beyond the group what's the best response? Should you leave the group, perhaps even starting a new group of like minded old grumpy bastards? :) Is it even ok to leave the group? Is there an expectation that you should contribute towards other members' growth, given what you've gained?
I suspect reputation has a large role to play in the latter questions. In real world scenarios, say you're training with a regular martial arts group, or you've been learning from a handicrafts group, you may feel an expectation to give back to the group in some way lest they think you a selfish leech.
But in the online world where its so easy to lurk anonymously who would even notice you leaving?
Finally, if the members don't feel any obligation to other members in the group, is it really a community?