My lowest resolution screen is my laptop (1920x1080), which is too big to run most apps sensibly in full screen mode.
For a tablet or phone, it works, but for a proper desktop, where I need to do work, it just doesn't work. I generally have 2 - 3 "reference" windows open, plus my "work" window, where I turn the reference material into a finished document. If it is full screen, I'd be switching back and forth between apps every couple of seconds! Way to kill productivity.
For browing a forum or social network or somebody who works in one or 2 LoB apps all day long, Metro might work, but for developers and information workers, it seems to be a non-starter.
Still waiting for the download, my laptop went to sleep and broke the connection...