Hidden features, reduced discoverability, cognitive overhead from dual environments, and reduced power from a single-window UI and low information density. Too bad.
Arrgghh. Jakob Nielsen is trashing Windows 8 now and that is funny. Funny because he does this by having not, 100, not 200 and not even 1000 people to try out Windows 8, he had the magic number of 12 people to take this test. Yes, that's right - 12 people and he trashes Windows 8 and this makes to the front page of Techmeme because guess what, Jakob Nielsen is THE authority on User Experience Design. Sorry state.