All the pretty of Vista with the UI responsiveness and memory overhead of Windows XP combined with the security of Mac OS X.
I know you don't like Windows, but you don't need to be so harsh.
Seriously, OS X's "security" is garbage (as of Leopard...I haven't touched Snow Leopard, but I doubt it's any better). The only way it could get worse is if Apple disabled system update by default.
I wouldn't really say that Vista is inherently unresponsive, either, you just need a beefy system for it to beat XP (2GB of RAM will do it). Windows 7, on the other hand, outperforms XP with only 1GB of RAM without compromising on the high-end systems. That's the beauty of it.
Oh, and there's the new taskbar. It's like a dock without the shitty parts (being unable to rely on muscle memory being the biggest one). Window peeking is also an awesome window management feature. Pretty comparable to Expos