There are some noted issues in Win7 regarding gamma changes in games and anything else that might require gamma changes. People, like myself, oldschool gamers who still run CRT monitors for 150ish FPS in games often run into darkness issues in nvidia drivers that need to be changed. In win7, you cannot do this. This makes anything that would require gamma changes nearly impossible. It forced me to revert back to WinXP.
This issue I believe only affects people with CRT's. However, it is noted that you still cannot change gamma in games if you wanted to regardless.
That is the only issue. I also noticed my FPS seemed a little lower in Win7 as well, but that was not the main reason I reverted back.