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.
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.