It's all around faster, not just in demanding games. Startup and shutdown are much faster, as are application loading times.
So is Windows 8. I've only found one single application which doesn't yet work in Windows 8 and it's a professional design program.
Windows 8 has been available since August. Any major compatibility issues are well documented by now.
This only applies to WinRT (not to be confused with Windows RT) applications, which aren't available on Windows 7 at all. Traditional Win32 applications run just fine.
There are some changes that take getting used to. It does require you to use the windows key a lot more.
That's not a bad idea. I believe that both operating systems can be installed for 90 days before activation is required.