> This page in a nutshell: When choosing an account name, do not choose names which may be offensive, misleading, disruptive, or promotional. In general, one username should represent one person.
Seems fine to have the sussinct description at the top followed by details on the same page. If there really are lots of people that have issues picking a username (these policies seem petty typical so I’d expect most users to be fine) then a link from the create account page would be a good idea.
For a few years the new username lists were trawled by vandal patrols and there was a lot of biting of newbies -- so much that "don't bite the newbies" had to be added to the policy pages.
For example: the section on "confusing usernames". This was added to avoid people suggesting they were a bot account if they weren't a bot account, or were an admin if they weren't an admin, or to prevent impersonation.
So, if you register as "kjwenflkjclnaksdnalmsd" that's confusing, but it's not against the policy. Except a lot of people reporting usernames hadn't bothered to read the policy, and so they were just reporting names like that as confusing. For sometime people using their real names in a non-latin script were being blocked because their name was "confusing". This again led to changes in policy.
What WP really needs to do (and what they've actually done) is have a bot that checks usernames and places them on a list with descriptions of the problems, and warnings about why it might not be a problem. (There are differences between "WhitePower88" and "MartyJenkins88") -- and then have people checking the list.