Windows 7 ~ Vista, as XP ~ 2000 ...
Windows 7 is actually still NT 6.x, based on the same code as Windows Server 2008 and, before that, Vista. It is the direct successor to Vista, with various compatibility improvements, although expensing security issues for compatibility.
This is not unlike Windows XP, which was NT 5.1, based on the same code as NT 5.0, Windows 2000. It was the direct successor to 2000, with various compatibility improvements, definitely expensing various security for compatibility as well -- although some were closed in SP1 and more in SP2.
One area where Windows 7 does something different is that it now ships a virtualization approach for NT 6.x so NT 5.x applications can run. Long story short, Microsoft "lost control" of their software codebase years ago, largely in the mass exodus of key architects, developers and engineers to Google and others late '90s through early '00s. So they will run a virtualized instance of NT 5.x for compatibility.
If you ever want to see the mess of conflicting issues between Windows programmer and binary interface calls, including Microsoft breaking its own standards and compatibility left and right between DOS 7 / Windows 4 (Windows 95/98/Me), Windows NT 4, 5 (2000/XP/2003) and, now, 6 (Vista/2008/7) -- the source code of the WINdows Emulator aka Windows Is Not Emulated (WINE), that allows non-Windows OSes running on PC platforms to emulate it -- is a great set of documentation of this. It is extremely flexible and very in-depth and involved because it lets you switch between emulating different Windows version -- or as they say, "bug for bug" differences in Windows versions. Of course, it's not good at running the latest stuff, because it takes years to catelogue all the changes, especially where things are conflicting or broken.
This is not unlike the Samba project, which emulates the countless variations in Windows Server Message Block (SMB) implementations, including some of the -- sometimes conflicting -- Common Internet File System (CIFS) procedure calls and what not. It's not uncommon for different Windows flavors, from applications to networking, to behave completely different, because there are differences in the standards it uses, let alone some bugs that are carried forward. Samba has found many false security and other holes in the CIFS/SMB protocols over the years, and I follow the project for this reason (to know where major security holes are in the protocols).
These attitudes and approaches, to not give a shit about compatibility long-term (let alone fail to document them), is the root cause why MS Office is not very compatible with itself over a couple versions. If anything, that's the main reason why I see OpenOffice.org adoption -- not cost, but long-term compatibility. Same deal on MS IE. I know a lot of corporations that are having trouble re-investing in the development of their MS IE 6-only sites, especially as they have issues with IE 7 and 8, let alone Internet standards and standards-based browsers.
After all, money is in the upgrades, not backward compatibility. So why bother if you're Microsoft or a Microsoft partner? It's very profitable to be a Microsoft partner for this reason. But at least I'll give Microsoft some credit for the NT 5.x virtualization layer idea, even if it's not implemented as well as it could be. It'll allow some support, although it will be exploited, much like the Office 11 (2003) compatibility layer in Office 12 (2007) has an issue with.
