Download the Crysis DX10 flyby, this is a good example of some of the benefits of DX10. Crysis will no doubt be THE DX10 showcase title when it is released.
Essentially, like SM3.0, SM4.0 is mainly oriented at performance and shader program versatility. Visually, geometry shaders are the main differentiator between SM3.0 and SM4.0, though we probably won't see the effects you can utilize with geometry shaders (some that you could already utilize with DX9) for a couple generations. Otherwise, it's fairly cut and dry. It's a revolution for how hardware manufacturers and developers can work, but for us, it's mainly about better performance.
I don't think FS X is going to change benefit much from the leap. The conspiracy theorist in me thinks that the existing version was released with intentional performance hampering devices in an attempt to try and drive eventual sales of Vista and consumer adoption of DX10, but that's neither here nor there.