Yeah, the things I noticed myself are partly those you mentioned already. Here some thoughts of me about it:
- Double item system. Probably dropping triple items in favor of that and keeping the star in your slot while you use it to make it more fair (imagine holding 6 red shells and getting another set of 3 right when collecting an item box - or using the star while being able to shoot away reds - that game will be more imbalanced than MKWii!). KartRider has done it like this since 2004 very successfully and I can only recommend Nintendo to copy that.
- Koopalings return (if they don't get removed like Kamek got in MK8, but I doubt it since that would affect a total of 7 characters at once) and King Boo is new. King Boo has a unique Circuit Special skin with gold plating on top of the front tires I think.
- The UI seems smaller? Running in full HD, resulting in smaller 2D overlays as they're just taken and realigned from MK8 for now?
- They took the time to create unique minimap overlays, the MK8 ones are light blue (simply reusing the graphic from the gamepad) and opaque, these are transparent gray. On a personal note, the character icons are still by far too fat to actually see who is in a group with you. I often don't even see players near me because I simply overlap them, it sucks.