Now it's extremely difficult to sing-up to Wikimedia projects.
With SUL and many existing user accounts most simple nicks and many combinations of nick and number are already taken.
Usually it happens this way (from many Wiki Education events):
User fills the sign-up form (with password and captcha). Most new users don't quite understand the need of unique login (many other services use email that is unique). They come with their name or some simple word as a nick. It's 99.9% already taken, but Sign-up form will not validate this immediately until they submit it. Now everything they filled in is lost and they are told that this nick is already taken. They come up with something more complex, but again most probably it's already taken too.

To make things more difficult there is another bug with signup (T116671) - after several unsuccessful signups due to already existing nickname MediaWiki starts to reject any signup attempts telling that source of the action cannot be verified and user should check if cookies are enabled (which are of course enabled). So from now on to sign up you need some trick like to use another browser (this idea will not come to 99% of new users)
So login can take up to 10-15 minutes and require knowledge of some geeky tricks, most newcomers can only succeed on this path if they are guided.

Ideally MediaWiki should also suggest variation of the nick that user submitted that is not already taken.

I talked about these issues on Wikimania with @MarkTraceur, and he said it's very simple and easy to fix. Unfortunately not fixed yet :(