Current usability issue: transports use different registration form layouts which will not fit well in some XMPP clients and which cause consistency issues. They also often use different labels for the same element which contributes to this inconsistency.
Possible solution: standardize the registration form to some extend, maybe also require transports to move all advanced stuff in an Advanced page or to Ad-Hoc commands
Current usability issue: an XMPP client cannot check requirements for field values. This could be useful for on-the-fly checks to change the background colour of a field with a wrong value to red. For example, on ICQ/QQ/Gadu-Gadu the username field may only contain numbers, an MSN address as well as for XMPP there needs to be an @ sign, a password may have a minimum and maximum length on some chat systems, and so forth.
Possible solution: create a XEP to send such information alongside the registration form.