This is coming up for me every now and then as a question from the perplexed.
It’s obviously not quite as obvious as it could be.
I tend to get this mostly from users who need to gain admin/editor/author access to a group blog created by somebody else. New users have to create a wordpress.com account to become an author, the administrator cannot just add them as a user manually as they can on a self-hosted blog. The usernames generated by wordpress.com appear to be getting longer and more randomly generated, which is A Good Thing for security but not so good for displaying on the blog, and people appear to be unclear on how to change the name that is displayed to readers of the blog.
Luckily editing your display name is pretty easy :-
The Personal Settings option in the Users section allows one to remove the visual editor option (for those who prefer to write directly in HTML), to push Twitter status updates to one’s blog, and more.