In all honesty, Vista was the first version of Windows that didn't feel as if it was, deep down inside, attempting to subvert every attempt I made to get my computer to work properly. I got a PC for the first time after Vista had launched, so I didn't experience any of whatever post-launch horrors poor drivers wrought, but it just doesn't have the glaring usability issues XP had. Having used OS X from 10.1 up to 10.5 ( I got Vista on a cheapo dell vostro to see if there was really any point in forking over the extra cash for a Macbook), Vista feels slightly more usable than Apple's OS. XP's interface, with a gigantic, multi-layered abomination termed the "start" menu, which ironically contains right above "start" the off button, is an abomination. Much like Finder in 10.1 and up to 10.3, it just isn't logical. I still believe that having an applications folder, as OS X does, makes far more sense than the Start menu, but with Windows Search/ Spotlight, I never launch apps from anything aside from the search bar. Vista also has nice networking, and WMP is a great media player, although IE 7 was as horrific as one could expect. Seven isn't much better, but it doesn't have to be. Apple's been getting money from us Mac users on an almost annual basis for upgrades to OS X, many of which (prior to 10.4, which is the oldest version of OS X one can really do anything on) were more like stopgap measures to fix the massive overhead that Aqua and other Apple additions to Darwin created (Hardware accelerated window management was also a huge improvement). Seven is a bigger release than the past two versions of OS X (10.5, 10.6) as it introduces a new UI paradigm that is just fantastic, and includes many fixes and little improvements. Windows search now has an autocompletion feature when searching through metadata (eg: type in date: and a calendar pops up), you can drag and drop things from the system icons thing, IE 8 is actually a good web browser (although is still no Opera), the libraries feature, media streaming over the Internet with WMP, managing the volume of each application, better automated driver fetching, built-in startup recovery, the ease with which protocols can be assigned to different programs, the improved language bar... Using anything else after 7 just feels illogical, although it does suffer from a poor placement of the taskbar; with wide-screen being the norm, it should be on the left or right hand side of the screen, as should the Dock. Also, XP is over 8 years old! It should have died a long time ago.
As to the nature of Vista, XP is broken. It is a relic of the past, before having a broadband connection was the norm. Install a RTM copy of XP on a system with an Internet connection that doesn't have a hardware firewall, and count the seconds until it's compromised. I'm sure that it was patched up through the years, but as I understand it, XP still starts everything as root. The whole non-elevated user processes idea is absolutely necessary; Unix and its derivatives have had similar security measures for years. That XP was somehow made usable through its stopgap patching is almost impressive, were it not so painfully hard to use.