It's my opinion that the Add User option in the admin interface should indicate which fields are required to make a user. A simple (*) with a note saying 'required' at the bottom would suffice, so that you don't have a suprise on the next page.
I hope this fits with the guidelines for bug reporting. I hope to help with WordPress much more in the near future, as I've recently converted from Simplog. I'm sure someone will straighten me out if this is an innappropriate place for "ui bugs"/"feature requests".