MPS and Endaar summed it up pretty well. Vista at home or at work is a joke, and I type this from a Vista laptop.
With XPSP3, I'm ready to work 10 minutes after installing it. Vista took me a week of configuring just to get it to reasonably behave.
Folders still don't remember view settings and any time a folder suddenly has more media files than documents, Vista assumes it's a media folder and blows up the icons to superfrigginhuge and sorts them by name. Folder sorting by date is a hidden option by default, and I have to set it for every new folder that I create.
Vista treats zip files like folders (just like xp) however when you install 3rd party zip software, it doesn't STOP treating them like folders. Open your rom directory in Vista's shoddy explorer and the tree will expand for miles as it treats every rom as a directory. You can stop it with a registry patch. Heck, you can stop anything with a registry patch, but it's extra work to do the same thing XP does out of the box, sensibly, the first time. I don't even get the option to click up one directory, i have to use the crappy breadcrumbs bar which uses relative paths instead of absolute paths, meaning it considers places like your user directory as a root path, instead of being located in /users/roaming/yournamehere.
UAC is a joke.
File writing and copying, even in SP1 is still terribad compared to XP. Terracopy helps this somewhat, but when I need an app to handle a basic GUI operation like moving a file, it's time to reinvest.
Managing networks is a disaster, and tweaking it to get a stable connection is even worse.
Half of the services you don't even need, and quite a bit of the new technology designed to speed things up, really just slows you down (I doubled my file writing performance just by removing my turbo memory chip!)
Interacting with open windows is less responsive that Windows 3.1. With Aero on, Vista forces the desktop to render with vsync on, meaning there are times when a dragged window has gotten as far as 2 inches BEHIND MY MOUSE CURSOR. Scrolling performance also suffers dramatically for the same reason. This poor performance gets passed on to any application you are running unless it's rendered full screen.
Vista drivers are still a mess. All of the copyright protection garbage that MS was forced into implementing makes it a pain to develop working drivers. Vista makes it so that applications have to be able to control your hardware, regardless of manufacturer so that you can't make copies of things that you already paid for. I'm using Asus sound card drivers on my Lenovo laptop right now because even after a year, lenovo still hasn't figured out how to write a sound driver for Vista that didn't stutter every 20-30 seconds. Put the blame where you want to, but at the end of the day it's XP with the hardware that works.
Vista eats a ton of resources for no good reason. Anyone arguing that every OS uses more resources than the OS before it, hasn't tried Windows 7 yet. It uses less than Vista, even before tweaking. XP used more than 2000 because it did more. Vista uses more than XP because...it's fat. yes, RAM is cheap, but just because you have more resources available, doesn't mean you need to waste it on a bloated OS. if you have $20,000, you can buy a sensible car that will last you years. If you suddenly get more resources, say, $50,000, sure you could buy a more expensive car, but you could also use those resources to buy the sensible car AND some sweet living room furniture AND even a new TV! With XP, you have more resources available to "spend" on other stuff. I want my 4GB of RAM to work for me, not just support my OS.
However! Vista with MAME is great. Why? because you're not interacting with Vista! Once you've set up MAME, you're interacting with your frontend, which communicates to MAME, and that's it! Vista64 is supported way better than XP64, and MAME64 offers significant performance advantages across all types of games.