From the ground up, Windows 7 is a refinement, not a replacement, of Windows Vista. Vista itself was aimed to replace Windows XP, much as Windows XP was meant to replace Windows ME and Windows 98. Since it is aimed to be a refinement, and not a replacement, there are many similarities between Vista and Windows 7. Many of the features that were first debuted in Vista, such as user account control, are being made easier to live with, in hopes of making the technological advances in Vista more appealing to the average user. These are improvements, not replacements or brand-new features. There are many places where Windows 7 looks better than Vista and feels easier to use, but many of the underlying goals and features are the same.
Ironically, the fact that Windows 7 isn't meant to be a replacement will likely result in broader appeal than what could be achieved in Vista. Vista was an important step for Microsoft, but new, different, and/or better features often conflict with older programs and drivers. Enterprise users have been particularly skeptical towards Vista for this reason, as most of them have business-critical programs that were made specifically for XP, and which would require a large investment in order to work out conflicts with the new operating system. Windows 7, however, doesn't change the rules like Vista did, and since Vista has been out for several years now, companies have had time think about implementing their existing software into a new operating system.