It's essentially impossible to build something as high-quality as Stack Overflow for an all-purpose Q&A site. The expertise required per category is incredibly high, to match the level of quality Stack has, and then the moderation has to be equally up to that bar. That has to occur for a thousand different categories (far more really, but you get the point).

So you need tons of users per sub, with very high level expertise; you need dedicated moderators with expertise in the category so they grasp what's what. And you need some way to bring all of these people in just to get the ball rolling (Stack did it by knocking over one category first of course).

Even Stack Exchange has failed to translate its very successful core site to a vast range of topics. If you look at where they've succeeded in a big way, it's exclusively tech-heavy or otherwise topics that geeks like.

I think Wikipedia has managed to be hugely general with a relatively high quality level so it must be possible. There might well be an underlying difference that can't be overcome but I can't see it from here.

Wikipedia is a terrible example to use for a community.

Wikipedia is a great guide of things to avoid.

Take signing up for an account: the software has some control over what username you can create. Then there is some level of admin on top. So, as well as the software limits there is the username policy.

http://en.m.wikipedia.org/wiki/Wikipedia:Username_policy

That page has nice advice about what to do with bad usernames - starting with do nothing.

There are two templates for bad usernames (templates are generally a bad idea) -- {{subst:uw-username}} or {{subst:uw-coi-username}}

Then there is a Request for Comments http://en.m.wikipedia.org/wiki/Wikipedia:RFC/N

And then there's an admin noticeboard http://en.m.wikipedia.org/wiki/Wikipedia:Usernames_for_admin...

But the noticeboard has two parts - a holding pen and the main board.

The RFC/U is the problematic part. Children rapidly post all possibly risky usernames as part of their gamified run to adminship.

You kind of expect editing topics like Palestine or Ukraine to be risky. You really don't expect simple uncontroversial punctuation gnoming to be horrible but it can be a really nasty toxic experience.

And despite all these problems, wikipedia is extremely useful.

What about all the Stack Exchange communities (e.g. Mathematics Stack Exchange, English Language & Usage Stack Exchange)?

"What's a list of good books about programming language X?"

"I'm switching from this programming language to that programming language; what are the important gotchas?"

Followed immediately by the question being flattened by administration as "not a good fit" despite the previous existence of these types of questions producing excellent discussion and answers.

I think SE is awesome in spite of its rules, not because of them. The whole "discussion is bad mmkay" thing seems to be cargo-culted by the staff of these sites despite many examples to the contrary.

While I agree these kind of questions requires answers with some information, in reality most answers would be highly subjective. Same subjective answers can readily be found by simple web search. They also promote me-too wars and pointless debates on why my subjective POV is better than yours. Soon these type of questions would overwhelm the "real" questions and the participants who want more substance would move on.

This has actually been my #1 appalling thing about Quora. The questions being asked on it are so often such a low quality that I worry about if I'm getting in to some sort of intellectual ghetto. Consequently, I rarely want Quora to tell anybody that I was on that website. I have also developed perception that people very active on Quora have fairly low bar (note that there are many tech celebrities on it but they are usually one-offs).

Examples from current Quora frontpage:

"How do people who have been affected by "Bendgate" feel about it?"

"Why am I so scared to ask for help?"

Personally, while I like some aspects of the community stuff around the different Stack Exchange sites, I feel like there's a lot of friction around a user jumping from one to another. It sort of feels weird, you know?

There are very different cultures on the different stack exchanges. I used to think it was a mistake to allow individual sites to have so much control over appearance, but now that acts as a useful cue that you're on a different website with different rules.