Shacknews quotes Relic as saying the DirectX 10 patch brings "improvements to model and map details, lighting, shadows, and overall image quality." If you'd like to see the difference for yourself, the folks at Game Almighty have put together an article that compares image quality and performance between the game's DirectX 9 and DirectX 10 code paths. The DirectX 10 code path indeed appears to make a noticeable difference in image quality, although on a system with a GeForce 8800 GTX graphics card, Game Almighty found that the DirectX 10 path more than halved average frame rates.
Users of Nvidia graphics cards who intend to install this patch are advised to download Nvidia's new ForceWare 158.45 beta graphics drivers. The drivers are available for both Windows Vista x86 and Windows Vista x64, and Nvidia says they bring performance enhancements to the DirectX 10 version of Company of Heroes. Nvidia's release notes don't mention any other changes over the previous ForceWare 158.43 beta release that came out last month, though.