ARS: Overall, it's pretty clear that Windows 7 is "Vista R2." Hell, the branding of the server counterpart is a dead giveaway here. Windows Server 2008 RTM was exactly Vista SP1; Windows Server 2008 R2 is exactly Windows 7. Why does one retain its branding but not the other? Because the Windows Server 2008 branding is popular and successful (the OS was, quite rightly, very well-received) in a way that the Vista branding is not. If Vista had gotten the reception it actually deserved, and become a brand worth keeping, it seems highly likely that the name would have been retained. And Microsoft knows it, which is why internally, Windows 7 is only version 6.1. Sure, the company has made specious claims that this is to avoid breaking applications with bad version checks, but the logic doesn't really hold; many of those applications are just as broken by "6" as they would be by "7," and if that were such a concern then the minor version wouldn't change either.
But at the end of the day, that doesn't really matter. Windows 7 is, overall, a fantastic OS. It builds on a solid platform, and just makes it even better.