The Win7 kernel is the Vista kernel with some minor enhancements and some pretty big fixes, which is why Win7's internal version number is 6.1, so Win7 is a fixed Vista, the rest is fluff, which they've improved as well.

Win7 is what they intended Vista to be but due to marketting they released it waaay early, given the financial situation they kind of had to take the bad rap to get the sales to finish the product (Win7)