For me, GMail reached an optimum in terms of user experience some three or four years ago. Then it was a mixture of changes that were nice but of minor significance (the look of the main interface is prettier now than those few years ago) and of "reverse improvements" of usability like very confusing and ugly icons, hiding core functionalities, all the "social" crap etc. I guess this might partially be the result of UI people coming and going to the GMail team, new people arriving always want to prove they can do better and do things their own way, and if things are already good, they can only get worse this way.
