Once some properly optimised drivers for Vista are available then performance in all areas will exceed XP. Vista is the first OS to have native PCI-Express support which means it can directly map the GPU RAM into the virtual memory address space. Making it also the first OS to support a 3 way paging system (page file, main memory, graphics memory). When people say "Oh Vista closes down all its bloat when you load a game so it runs fine" what they really are saying is this: When you load a game in Vista, all of your desktop textures (yes everything on the desktop is stored as a texture on your graphics card) is paged out of the graphics memory into main memory. If the memory manager decides main memory is quite low then it will actually page them out one stage further into the page file. This has then freed up a lot of the graphics and main memory for the game (or some other big processing power application). The advantage with this approach is that only "enough" is freed up to support the game. So it not only maximises the performance of the game but also other running apps - given the conditions.
Secondly is the VDDM (Vista Display Driver Model). XP was still using the display driver model as introduced by Windows 95. In short it was crap, slow and tiresome for developers. The new model hooks very tightly into the NT kernel (which by the way, DirectX 10 has very tight hooks into the kernel also) maximises performance and reducing overheads.
Now onto security... There is a common misconception going around that Vista is a "complete rewrite" of Windows (probably due to the time it has taken to get this far...) This is simply not true. Windows NT has been in development since '89 and a complete rewrite now would take Microsoft many decades. Vista, having only taken about 4-5 years so far, is no where near that mark. Yes very important parts of the OS have been redesigned for the future but this does not amount to a total rewrite. Take the networking stack, this has been a 100% rewrite. Microsoft seized the opportunity because it knew its old stack was not going to scale well for the multicore future, nor was it fully IPv6 compatible. The new stack is by design more secure, faster and more reliable - as well as having more features like better QoS. Now of course there will be bugs but seriously, who runs without a NAT router in the way these days anyway? Longer term the new stack will show its fruits.
Another new part is the audio stack. Completely redesigned. Remember when Creative used to release those audio drivers that blue screen'd your brand new XP PC? Well that can happen no more. Audio drivers are now debundled from the kernel, they are a user mode process! So when Creative mucks up their driver rollout for Vista - at least it won't blue screen your new Vista PC! Infact other than a slight pause in your audio whilst the user mode driver is restarted automatically, nothing else will happen!
I hope this gives people at least a small insight into why Vista won't suck
