They're not. WIndows 7 is Windows Vista 2.0, just a different name so as to avoid the (ignorant) bad press of Vista.
UAC isn't useless - were it linux, you'd be typing a password every time. And as a non-administrator user, you DO type a password every time.
Vista's security - out of the box - shits all over any version of Windows previous. They've actually thought about the design of the OS with security in mind this time. Performance .... well, run it on the appropriate hardware and you'll have wonderful performance. I know I do. I don't have what would be considered a top-end beast anymore.
As for compatability, the reason for incompatability most of the time is that previously, the Windows system folder and Program Files system folder were whores - open to everyone. In Vista, they're prudes. Which is great from a security perspective. Unfortunately, software developers didn't want to change the way they've always done things, so things broke.
That, unfortunately, is the name of the game in marketing. It's all one-up-manship and me-too-ism.