The actual driving is superb. The controls are superb. The tracks are superb. The online play is superb. I'm sure other reviews cover that better than I. What I want to focus on is what I feel isn't so superb. This is my opinion, of course.
1) Items. In my opinion, the balance between skill and items is exactly where it was in 7. Mario Kart 8 did not move the balance towards skill whatsoever. There is still no practical defense against the blue shells that come at the same frequency they did in 7 (speaking from my personal experience). If you get the useless coin from a box, you can rest assured you will soon be losing places to a red shell. Badly-timed incoming items can still send you from 1st to 8th in a hurry.
If you liked the skill-item balance in 7, you will like it in 8.
2) Battle Mode. The Gamespot reviewer covered this better than I ever could, considering I've never played it. =p. May as well be missing.
3) Mission Mode. It is missing. WHY?! It was the most fun part about the DS Mario Kart. Even the Sonic MK rip-offs have it. Note that I think the Sonic MK rip-offs (especially the second one, bleh) are no where near as fun as MK, but both of them had Mission Mode.
4) Clock-timers are missing. WHY?!
5) I too am kinda disappointed in the character roster. I spent many games as Skele-Koopa in the Wii version.