The labels for input boxes are hard to read for those with vision problems (low contrast).
The labels disappear and input boxes rearrange themselves as you enter things into them (confuses the elderly, the non-tech savvy, and trained interface designers).
The virtual windows hide a number of things that used to be "one-click" under buttons which now require multiple clicks, and certain keyboard shortcuts no longer work (compose a new message with CC/BBC already revealed).
Thanks to Daniel M. Reck for this suggestion.