There are two downgrades: no headlight shadow casting during night stages, and the fact that the stages are created by a stage generator, so no masterfully handcrafted stages this time (not that they're bad in 4). That aside, there are a lot of improvements, from road surfaces, shaders, etc...
Just want to thank you guys for your pix. Honestly, I only visit this thread during steam sales to get purchase suggestions. Nothing says "take full advantage of your graphics card" like 4k screen shots.