have you ever seen that new GUI in action ? it is nice, it is smooth (no flashing refreshes or whatever) and it is powerful ... just moving your cursor around the "dockbar" will prove you what the system is capable of.
You like to see display glitches when scrolling a window ? i don't. And that's something i've never seen on OSX (i don't own one. A friend of mine do
What's the big stuff about OpenGL displaying the desktop ? it's portable (OpenGL is standardized, unlike M$DX), it's powerful (your GPU is now used even if you don't play Quake
and unlike the pittiful 2D acceleratorz, it can extend to whatever-your-mind-can-deal-with toyish special effects ...