That's an interesting observation.

The problem -- if it is one -- is certainly not limited to the Win 8 Start menu. There are plenty of interfaces where pressing a button, or some similar action, pops up a new full-screen or nearly full-screen UI element that looks & works differently from what was there before.

I wonder whether any research has been done on whether this makes any significant number of users forget what they were doing. If not, I think such research would be worthwhile.