1. Soft particles with particle physics.. in combination with a good weather system.
2. Global Cloud Shadows and volumetric clouds... especially on windowed skyscraqers this would look really awesome.
3. Volumetric light.. i know its already in, but i think with dx10 it could even look better than on Dx9.
4. Same goes for volumetric fire.
5. Dynamic volumetric smoke
6. Dynamic wet surfaces and water splashes ( in rain levels)
7. Soft shadows
8. Shader Model 4