Age of Conan's Testlive server is now running a version of the game that can output DirectX 10 graphics, a feature that many people missed at launch. The current implementation probably isn't indicative of what will end up on the Live servers, but if you just can't wait and want to see the work-in-progress, Age of Conan Vault has a number of DX9 to DX10 comparison screenshots on show right now. The test client as-is doesn't have some of the cooler features like volumetric fog or windy trees just yet, but these would be better seen in video form anyway.
The most stand-out difference between the screenshot pairings that we can pick is the upped brightness and visibility in dark areas on the DX10 side. In some (but not all) of the shots, the textures look noticeably better too. The AoC Vault reviewer experienced a significant FPS drop, from roughly 50FPS down to 20FPS, when switching from DX9 to DX10, but this is apparently not that common among other testers -- most do get their share of graphical glitches and crashing though. Hopefully it's all ironed out when Funcom decides to push DX10 to the live servers at a later date.