There is a substantial graphical improvements as far as special effects, dynamic lighting, transparencies (maybe even improved multi-layered skybox as used in the original Unreal), reflections, etc by playing with glide support. This was how the game was meant to be played. http://www.1up.com/do/blogEntry?bId=9091474
D3D makes things much darker, and light sources are bland, d3d version is overall aesthetically bland in a game engine that's already showing its age this doesn't help much.
The same goes for the original Unreal game, which shares the same engine. It was designed for Glide in mind. There were areas reflections that were truly something to behold, that one loses by playing direct 3ds/open gl modes.
It's a shame that no one has modded a glide to directx10/11 driver, like they did for Deus Ex which access all the bells and whistles of glide-only effects but taking advantage of modern system. Deus Ex was also built on the Unreal 1 engine, so I'm not sure it would be all that difficult to make the modded graphics driver?.
Anyone see if the DirectX 10 renderer works with Undying? http://kentie.net/article/d3d10drv
(note the multi-pass through multi-layered transparent skyboxes in Unreal!)
There are apparently enhanced OpenGL renderers that try to do the same thing (but doesn't emulate all of the improved effects that the directx10 renderer allows for); http://www.cwdohnal.com/utglr