Regardless of what people say, Vista was a big step forward from XP. Many people held on to using XP (some even still do), but after using Vista and now Windows 7, almost exclusively since Vista was released, going back and using XP feels very dated and old fashioned. Also Windows 7 doesn't feel that different to Vista, just a little cleaner UI, a little smoother and faster to use etc but people slag Vista off a lit and praise Win 7 at the same time. The reality of that is exactly the same as when XP was first released. When both XP and Vista were launched the average home PC was not really up to the spec needed to run them well. With XP it really needs 1GB of ram and a 2GHz+ CPU to feel smooth, responsive and nice to use. At launch most people had 256MB, and luckier ones 512MB ram. This gave people a really negative impression of using XP as it ground to a halt resorting to harddrive caching to compensate for the lack of ram, and with hdd's not being so fast in most people's PCs at the time didn't help either. And with Vista it was the same. You really needed 2GB of ram, a 512MB or more dedicated GPU, and a 3GHz CPU. Most people trying Vista at launch had their XP spec PCs with 1GB ram, a 2-2.5GHz CPU, and worst still many with more budget PCs and Laptops had built in graphics that shared system ram and were not good. With these setups Vista just ground to a halt. Throw it a dual or quad core CPU, 4GB ram and a 1GB GPU and it was very nice to use.

As for the actual main reasons for professional. To access more than 4GB of ram I needed a 64bit OS, and XP Pro x64 was completely rubbish with hardly any driver/hardware support and very limited compatibility. Vista 64bit fixed this and allowed me to run 8GB ram for video editing (which is the one area you really need every bit of ram you can throw at a system for ram previews during development if a sequence). Also Adobe started to drop support for XP and some of the software in their complete Master suite was starting to state a minimum of Vista to work. A less important reason for upgrading to Vista was DirectX 10 support for newer graphics cards, and games directly supporting it. With XP people are stuck with DX9.0c. And with 32bit OS's you have the 4GB ram limit shared between the GPU and CPU, so if you have a current 2GB GPU are are going to find only about 2.5GB of system ram available, regardless of how much physical ram is installed.