NeoNight,
The courses are smaller because they dispensed with the almost unbearably huge-as-fuck Wario Stadium and Rainbow Road. (editor's note: I -love- Wario Stadium, and have a 100% first-place ratio in that race against anyone who does not use the shortcut)
The speed feels just like MK64 to me, I dunno... I had MK64 and DD both hooked up on two different TV's at exactly the same time, the speed and handling are nearly identical. The only noticeable difference in handling is entering powerslides - you have to 'turn' more to activate the slide, whereas in MK64 you can just jerk the stick to one side for a split second and hop.
And just for additional reference, in MK64 I use Bowser, while in MKDD I use the two Koopas.

They obviously changed the overall handling (I mostly can't get used to power sliding using heavier vehicles), but WHY IS IT WRONG TO "UPDATE" A GAME FOR ITS SEQUEL? Even
MADDEN gets updated gameplay on some level.