Mail's UI is appalling full stop. Apple is not innovating, they are messing around.
To quote John Siracusa:
'The first time I saw the new Mail toolbar, I filed a bug on it. (Radar 3968093: "Toolbar buttons in Mail 2.0 are hideously ugly.") It was immediately closed as a duplicate, so at least one other person agrees with me.'
Huge UI inconsistencies are what I pay extra to avoid when I use OS X (on Apple hardware)...