I've used the latest version of Windows in Polish and English for fair periods of time now, on current hardware, with the results being between bearable and annoying. Even when it's been usable I've been aware that XP on the same hardware would be far more responsive. My point here is simply that "mostly adequate" isn't the type of sobriquet a 'latest, greatest' operating system should be aiming for. This was never an issue moving from 2000 to XP, making Vista the biggest stink exuding from Microsoft since Windows Me (Millennium Edition).
UAC (User Account Control) — whilst something I'd turn off if I was the main user of a system — doesn't seem to work too badly for people who don't install software or change settings; but there's no sane reason the location of many settings has changed. Someone new to computers and learning ways of doing things for the first time won't notice this, but those who've used them much at all in the last five-to-ten years certainly will, and it's more pronouncedly more noticeable using a machine in a language you don't speak. (Something that hasn't been as tricky as you might think, for the last decade or so. Some things you remember, but a lot is actually intuitive.)
The strongest impression left is sluggishness — Windows still can't accurately calculate the time remaining to copy a few hundred megabytes of files, but slow copying in general has been widely reported by users. Apparently it's being addressed by the upcoming first service pack, rather missing the point that Vista should never have shipped with such a basic feature less than optimal. And the service pack doesn't appear to improve general system responsiveness to anything like XP levels...
The eye candy can thankfully be turned off, though since eye candy and DRM seem to be most of the new features users will notice this is rather a good argument for XP over Vista. Unfortunately the layout of applications suits the Windows Classic appearance less and less these days, which is a shame for those more interested in doing things with their system than looking at shiny widgets. And I freely admit to being the type of philistine who prefers user interfaces to be neat, simple, functional and consistent rather than a soup of colours, gradients and blends.
The most damning thing IMO is that Vista doesn't seem to be judged by much of the pirate community to be worth copying illegally. If people won't snap it up for free but would rather download XP (and don't fool yourself it has anything to do with copy protection; that's been more-than-sufficiently circumvented by now) it's a poor reflection on Vista, and particularly its viability on current hardware.
Even assuming people are happy buying more powerful and expensive computers for Windows to waste resources, compare the retail price of Vista Home Premium (the cheapest version with the flashy new Aero interface that Vista is being marketed on the basis of) and the newest darling of PC hardware reviewers, the Asus Eee PC — yes, for the price of just the normal current Windows operating system (£200 or thereabouts) you can buy an ultra-portable laptop with solid-state storage, built-in wireless networking, decent office suite / web browser / email client. Unsurprisingly, it doesn't run Windows — because Windows would be a huge part of the cost the manufacturer would then have to pass on to customers, even at the discounted price Microsoft offer to system-builders.
Those looking for XP systems can look to Dell's small business site. You don't have to be a business to order from them.