It's generally more user friendly for consumers overall. And glossy!
It makes more of an effort to helpfully solve problems, although sadly in typical MS style it usually fails miserably. Take the "Windows is searching for a solution..." thingy that pops up when an app crashes - sounds great! except that in over a year of using Vista, despite it "searching" many, many times, it's never once found a solution for any of the crashes I've endured!
It's definitely prettier, the little sidebar gadgets are quite nice, the funky 3D window switcher is undeniably cool but nothing more than a shallow gimmick really.
There's lots of little changes, relatively trivial in their own right, but ones which I do start to miss when I'm back at work using XP all day long.
File searching in Vista is immeasurably better than the god awful excuse for a search found in XP.
Sadly I've not found Vista to be any more stable or better at resource management than XP. It's still very easy for errant applications to bring the OS grinding to a halt, and frequent reboots are still the order of the day.
If I were buying a retail OS for an existing PC, the choice between Vista & XP would be a close one. However, for a new PC with OS installed, I'd probably opt for Vista - in theory, you avoid any installation headaches, compatibility issues should've been addressed by the retailer, and whether you like or not, Vista is the future of Windows, MS won't support XP indefinitely, and I'll bet they're spending a lot more on Vista R&D than XP at the mo...