There's none, it's another product from the giant money hungry Microsoft.
Not really (BUT MICROSOFT IS STILL EXTREMELY MONEY HUNGRY, like Uncle Scrooge McDuck). If we see in a way, Windows 7 is better because:
Simple Interface, very, very, very easy to use, and beautiful (and doesn't suck up alot of CPU Usage)
Better security and it isn't as annoying as Vista's
DirectX 10 or 11
But it has it's holes:
Bad old game compatibility
You need alot of RAM to run some stuff good D:
Lag if your system is a bit outdated (mine is in RAM and Processor, as for GFX Card it's still great...)
SOME GAMES ARE ONLY COMPATIBLE WITH WINDOWS VISTA/7 like Just Cause 2

Still, my primary OS is Windows 7 and secondary Windows XP, even still I voted for Windows XP...