wikipedia:
New features:
* Fixed pipelines[9] are being done away with in favor of fully programmable pipelines (often referred to as unified pipeline architecture), which can be programmed to emulate the same.
* New state object to enable (mostly) the CPU to change states efficiently.
* Shader model 4.0, enhances the programmability of the graphics pipeline. It adds instructions for integer and bitwise calculations.
* Geometry shaders, which work on adjacent triangles which form a mesh.
* Texture arrays enable swapping of textures in GPU without CPU intervention.
* Predicated Rendering allows drawing calls to be ignored based on some other conditions. This enables rapid occlusion culling, which prevents objects from being rendered if it is not visible or too far to be visible.
* Instancing 2.0 support, allowing multiple instances of similar meshes, such as armies, or grass or trees, to be rendered in a single draw call, reducing the processing time needed for multiple similar objects to that of a single one.[10]
for exampel:
http://en.wikipedia.org/wiki/Geometry_shaders
you can't do this with your DirectX9...