I have owned the old 50mm 1.8D and currently own the new af-s version. The new version has faster AF, less noisey, no extending/rotating of front end, has dust/moisture rubber seal on the mount, and the filter thread is now 58mm instead of 52mm. It also now has the addition of an aspherical element.
I love the lens, but can't really say there is a significant difference between the two in real world/normal day to day shooting. I would say that the new version has more "pop" to the image. Digitalrev did find that the new 50mm 1.8G is sharper in the corners than the 50mm 1.4G.
I chose it because although when I owned the old 50mm 1.8, it could AF on the D700, but now with the D5100, it wouldn't. I didn't want to pay the extra for the 1.4 so I got the 50mm 1.8G instead.
All I can say is that if you buy the new version, you won't regret it. It is my main lens on my camera.