I joined Quora soon after it launched, thought it was kinda cool, forgot about it, and yes, then rejoined again sometime in the fall of 2010. There was a nice linguistics community built up for a while (I'm a linguistics undergrad) until everything kind of fell apart. Quora didn't know how to separate people very well, so people interested in memes gradually crowded everything out.

BUT I didn't actually have any problems at all with Quora (at least, nothing notable) until the launch of Boards. In fact, I really enjoyed the linguistics, philosophy, chemistry, biology, theology, travel, and cooking communities. I mostly ignored the Quora superusers and focused on niche experts, like you mentioned, but eventually it was just too much.

Perhaps I will revamp my profile when I get the time, start from scratch, and things will work out better the second time around.