I spend ages looking through the buttons trying to remember where the one is for creating a new message. It seems arbitrary what's a button and what's a link. Grouping of 'concepts' seem utterly arbitrary (notably the list of choices presented in the dropdown next to 'reply' in a message chain).
I just don't think I've ever been hugely impressed with googles UI credentials - everyone always seems to put them up as some kinds of genius in terms of usability but I just don't see it.
Ok, google maps is a notable exception.