Well, initially XP was 2k in bad. They fixed that with XP SP1 and by SP2 it had enough staying power to compete with the next two Windows versions.
Then we got Vista, which was bloated as all hell, had more compatibility issues than early XP and gave us joys like UAC, which is kind of like gksudo or OS X's admin password dialog except that it takes ten seconds to load, tosses up a modal dialog that blocks the entire desktop and occasionally makes the modal dialog appear to be on top of the other windows while actually placing it behind them, leaving it (and the application that triggered UAC) unclickable until you bring it to the front. On the plus side, ctrl-alt-del became much more powerful, capable of breaking out of misbehaving programs that would've prevented access to the Task Manager in earlier versions of Windows.
Windows 7 is essentially what Vista should have been at launch. Many of the worse kinks have been ironed out and you can now change the network setup (such as reordering NICs) without rebooting, which is very welcome. Few complaints here except for UAC still taking ages to load. Privilege escalation is not a trivial task in Windows-land, it seems. It's certainly not as easy as "verify user password, confirm that user is in appropriate group, become root". Oh, and Windows 7 revamped the VFS, making it a bit convoluted. Still, it's a fairly solid release.
Windows 8 assumes that everyone uses a desktop with a touchscreen monitor. If you don't use that configuration parts of the UI won't work particularly well. The Metro UI (or however they call it this week) is built around touchscreen gestures while the desktop mode still assumes that you have a mouse and can perform precise clicks with at least three buttons. Oh, and no start menu; you're expected to use Metro instead. There's a reason why they're talking about adding a start menu and a "boot to desktop" option to the next Windows.
As you can see, Windows release quality got really spotty after Windows XP. It's no longer a question of how big an improvement the next version is; these days you consider how long you can possibly last with your current setup because half of the new versions are severely unappealing. Of course it doesn't help that Windows seems to have run out of killer features as far as the ohme user is concerned. Vista gave us window tiling, 7 gave us "now with 80% less horribleness" and 8 gave us a user interface that virtually no computer on Earth is really compatible with... and the killer feature for 9 seems to be "we removed Windows 8's killer feature".
Sure, there's new DirectX versions but many people don't even care to do the research neccessary to notice the difference between DX 10, 11 and 11.1.