Generally a lot of people frown on Vista because of how stressful it is to game on it. For example, while it would take XP/7 1GB of RAM to play a game on recommended settings, it would take Vista 2GB of RAM. Or perhaps a couple more tenths of a ghz or even more than that to run a game in contrast to XP/7.
Also, it's a little slower than Windows 7, therefore people are speed demons. No matter what.
I ran Vista for awhile and I had no problems with it, I only upgraded because I'm a PC gamer and Windows 7 was eagerly anticipated by me.
edit: I was ninja'd