Doesn't seem all that different to me. Levels are still stupidly big and confusing, totally open to cause every software renderer error in the engine, and have no easy way to signify what's going on.
I found a simple way to make the stages more fun, though. I took WadAuthor, hit Select All, right clicked, Scale Sector, set it to 50%, and hit enter. Saved, nodesbuilt, and ran.
Here's Luigi Raceway with that simple thing done to it:
http://mystic.sepwich.com/luigiraceway.wad
Suddenly, the stages are fun to run through, there is stuff to see and do, and almost all of the graphical glitches are totally gone. I did it to most of the other early stages with the same effect. Every time, the map became much more fun with about 15 seconds worth of work.
While shrinking the maps deals with the size problem, it doesn't seem like it would deal with the confusing problem, but in many situations it actually does. Because all of the renderer errors are nullified, it's much easier to see around the stage and figure out what the player is supposed to do. When I can see my entire objective in front of me, it's much easier to analyze it and figure out the solution.
The bottom line is that almost all of the levels are just TOO big. They're totally open, with absolutely nothing to see or do. The path stretches forward totally flat with nothing on it for miles. Levels aren't supposed to be like that. They can stretch for miles, but there has to be something to see and do there. Add scenery, add turns and stuff to do in long stretches. But whatever you do, don't leave them totally flat and barren. I got bored before even finishing one lap on most of the courses before I shrunk them.
Sonic and Mario Kart are both about flow. Getting lost in a sea of confusing labyrinths and renderer errors kills any flow your level pack has.