You know, there have been a few comparisons to OS X in this thread. I've used a decent amount of Lion, on a laptop with a multitouch trackpad, and it makes HEAVY use of gestures. I wonder - just how useful would Lion be without said trackpad? Things like two-finger-swipe to switch full screen apps (or "desktops" as they call them), or three fingers up to get to Mission Control, or four finger pinch to get to LaunchPad - I have to assume there are ways to do those things with a mouse, but what are they? Do you have to pull up your dock, click Mission Control, then select the full screen app you want to move around? That doesn't sound any more efficient or intuitive.
I really don't think the comparison is as invalid as some are suggesting - its just a more gradual, subtle change rather than doing it all at once.