Disappointments so far:
-can't hold two items at once
-can't use gamepad as rearview, as far as I could figure out (I mean besides pressing X)
-I haven't found a good kart for snaking yet (low handling and traction, high acceleration, I think)
Course design - especially aesthetics - is remarkably good though. Still can't compete with N64's Star Wars Episode 1 Racer, but you can see where most of the development time and effort were spent.
More disappointments:
-They advertised editing videos quite heavily, but it turns out it's an incredibly weak system with just a few settings and no actual control of the camera, timespans, or anything else you'd actually care about.
-You can't play while uploading videos. Be ready to stare at a frozen screen (or go do something else) for 5+ minutes while you upload a 30-second video.
-Acceleration and Handling stats are intimately linked. MK7 did this too IIRC. There's not a single character or kart part that increases one and decreases the other. Maybe this was meant to reduce the impact of snaking (or other well-thought-out uses of stats)? Make sure there aren't any good karts? (Specifically, I wish I could get high acceleration with low handling.) What they have instead are a bunch of identical and redundant parts.