Saying Vista works fine on your machine isn't good enough of an argument. It is common knowledge that vista has resource allocation problems, you shouldn't have to have extremely fast RAM to just run the OS.
Most of my problems are annoyance problems, I get tired to telling the OS 3 times that I agree to do something such as open a program on my computer. On Destops, it takes a long time to start up. On laptops, it takes a long time wake up.
On my mac, it is 3 seconds after I pull the screen open that I am surfing the internet. 2 of those seconds is because the computer is still connecting to a network. When my mac is in hibernating, it uses almost no battery power either. None of these things I could say for any PC.
I liked 2k the best. Had great options for networking and mainframe maintenance, obviously not needed for most. For ease of use and speed, I like XP just fine.
Once you start video/photoediting, vista quickly shows it's weaknesses.