I, for one, am very glad to see Microsoft attempting to innovate their UI. I'm not going to get into a war about who stole which ideas from whom. I am just happy to see that MS is really trying to do something different with its UI -- really the first major change since Windows 95. The old one isn't working any more, and it's been time to move on for 10 years.
However, it may be too much, too fast. Doing a complete and total paradigm shift in one step is likely to offend many Windows users. I don't think that even Apple would have gotten away with that in the switch from OS 9 to OSX if it weren't for the fact that their user base was both small and fanatic at the time.
I expect for Lion to be a step in the direction of a new UI paradigm (one that is more touch friendly), but I think that the entire transition to the new way of doing things should happen in several steps so as to not be too jarring for the less adventurous.
My hunch is that many will jump to Mac OS if such a radical change is made to Windows in one step because Windows will no longer be familiar territory for those who are hanging onto it for only that reason.