1. IIS 7 Allows Multiple Web Sites
On XP IIS was limited to one web site. To develop multiple web site one had to use virtual directories or develop solely with Cassini (VS Web server). The problem with both of the aforementioned strategies is that they create a significant difference between the development and production web servers. I prefer to develop with IIS, and Vista allows me to do that.
2. IIS 7 Plays Well with Asp.Net MVC
I love Asp.Net MVC but it really does not work well without IIS 7.
3. Vista works with my hardware
Most of my development work is done on my Dell Inspiron 6400, which had loads of driver issues that I never resolved when I was running XP Pro. Vista Business just works.