One of the big UI issues of recent note has been the GNOME Spatial nautilus change. Gnome Developers broke the consistency rule. As a result they angered and confused some of their current users. The issue wasn't really whether or not Spatial nautilus was useable or not it was that the users suddenly had a radical change in a crucial feature of their UI. They would have been better off phasing it in slowly. Make it a feature not turned on by default. Then the first time a user turned on Gnome after upgrading he still felt at home. He could turn it on later but the initial shock of "Whoah!!!! what did you do to my file manager" would have been avoided. Most "users" don't like being shocked or surprised by their applications.
Consistency up front and flexibility can be taken advantage of later.