WDDM allowed for some improvements but it was at the cost of compatibility.
Now, here we get to the crux of it.
There are several people in this thread complaining about things that Microsoft hasn't fixed -- but
could if they so choose. And look what happened when they took something that was fundamentally broken and
fixed it? We get people complaining about "some improvement but at the cost of compatibility."
Well, freaking duh.
WDDM gutted GDI, which had an epic pile of flaws. WDDM removed 3rd-party developed drivers from ring level 0, which drastically reduces stability problems that Microsoft cannot otherwise control, or see or effectively change (but are ultimately still held responsible for.)
Doing these things required new driver model, new kernel interfaces, and a LOT of work. Backporting to XP could've certainly happened, say as part of a whole new service pack. But then we'd be also needing a whole new slew of drivers at the same time. Talk about a huge pile of fail when you take an already functional OS and replace such a massive chunk of it, and then require all the 3rd parties to follow suit.
How long did it take ATI and NVIDIA (primarily, NVIDIA) to get really well functioning drivers under Vista? Almost a year, wasn't it? The public would have accepted this change in XP like a turd in a punch bowl at your local Sunday church luncheon. Seriously.
So either shut up about MS not changing anything, or shut up your complaints about them changing things. I'm tired of reading / hearing / seeing people bitch about what doesn't get changed, and then when something
does change, they bitch more. It makes anyone pulling this nonsense look entirely ignorant, which apparently, and I see several instances of it right above me.