I would agree that most of the tracks in DD and Wii are a lot more interesting than most of those in 64. Those in 64 are mostly pretty flat (excluding pits) and didn't tend to have much going on. This is even more stark when comparing equivalent levels on the games.
For instance the road level in MK 64 was a simple loop road with a bridge and some cars. There were no atlernative routes and really nothing there besides the cars. DD had two road levels. The first had several shortcuts, including a pipe that shot you out, a sidewalk separated by a railing, a bridge you could drive up the side of, and a couple tunnels. The second had the track split into 3 intersecting paths, a looping on-ramp, and a pit.
Similarly, the snow level in MK 64 had an iceberg in the middle of the road at one point, some pinguins, and a cave with some pillars. The one in DD had ice-skating shy guys in place of pinguins, dudes that froze you solid, a couple of blocks of ice with no traction, a much larger downward-sloping tunnel, and a couple of areas you could cut across by going over hills. The Wii one has a launcher, skiing shy-guys, moguls, jumps, a pit you can jump across, deep snow, and is one of the steeper down-hill levels in any MK game (and is down-hill almost the entire way).
The MK 64 seaside level has a hole you can jump through and a ramp, while the DD one has areas that are flooded by tides, a pipe that shoots you out, a big hill, enemies that chase, and a big jump, and the closest one to that on the Wii has a river you need to drive down with several cut-offs, pipes that shoot out shells, an underwater tunnel with water and lightning bolts that shrink you, and a watefall that tries to push you off the course.
Even the same-named level, Moo Moo Meadows, on the Wii has a decent hill, cows on the track, a field you can cut across, and an area with grass patches and moving moles you need to avoid. The MK 64 one has some fixed moles in a bridge you have to avoid the pillars of, that is about it.
The standard rainbow road level in MK64 is basically a road with hills, a railing, and chomps. The DD one has a launcher, lots of sharp turns with no walls, much larger changes in altitude, and lots of boosters. The Wii one has several launchers, several splits in the road, a part the road that bounces you in the air, lots of boosters, lots of areas with no walls, and an area with pits you can jump over or walls you can shoot up.
I would say in pretty much every case, the equivalent levels in DD and Wii have at least as much, and often far more, going on in them than in MK 64, and almost every level in DD and Wii have at least as much going on as the most complicated level in 64, and most of the levels in MK 64 have a lot less going on.