Anyone who's actually been following Microsoft's rather frank information on Windows 7 understands that Vista was the code-base overhaul and that Windows 7 is not and was never advertised as repeating this exercise. Neither OS X nor Linux variants revamp their entire kernel architecture at each new release. However, Windows 7 does update their kernel with a new modular structure(min-win) that allows it to be more resource efficient and for unneeded portions not to load. Other aspects of performance have been gone over with a fine-toothed comb, such as windows not currently visible not consuming machine resources. The GUI has also been overhauled for usability, including fine tuning of user access control. There are at least as many updates and improvements in this OS as users are accustomed to with major releases of the other two major competitors.
"Everything should be made as simple as possible, but not simpler." -- Albert Einstein