It seems like the number one complaint so far is "It's different, and I don't like to think". That's just lazy, and I tend to discount it immediately.
There are two fairly valid criticisms, however. The first is that by moving functions into various gestures and hidden panels, the discoverability is quite poor. I'm constantly forgetting that the search feature is buried in that "charms" bar, and instinctively look for a search field on the screen somewhere. I'm sure the Microsoft knee-jerk approach to "fixing" this will be to print tips and reminders on the display bezel, which of course won't make any sense when the screen is rotated some other way. Going back to the drawing board and completely re-engineering a concept doesn't seem to be their thing.
Second, the weird desktop/tablet UI dichotomy is baffling. Functions that were previously confined to a small number of places - chiefly the Start menu and Control Panel - are now spread across two "control panels", a hidden "charms" bar, a "Settings" button in that charms bar, and many of these functions bounce back and forth between the tablet or desktop UI, or even duplicate features of one another. Key functionality has also been removed entirely. Where does one view, edit, and reorder the entire list of saved wireless connections? Nowhere, unless you want to use the netsh command!
So while I can appreciate making finger-friendly design considerations, the way they've done it is disjointed and nonsensical. If I had to fix it, I'd allow "Metro" apps to run windowed instead of only full-screen, make it easier to scale up UI elements of "desktop" apps for touch use, get the Control Panel consolidated into a single point of access, and put some of the most common features of the old Start menu directly on the new one, without hiding them off-screen or in menus (Control Panel, Devices and Printers, Run, Computer, Documents, etc). If you change the window manager to act more like the Metro mode when a window is maximized, then you've got a reasonably successful marrying of the two concepts.
For traditional desktop use, it's not at all horrible for an advanced user, and does have some nice performance and usability improvements here and there. For casual home users, it will probably be overly confusing, and leave them shopping for iPads even more than they are already.