Alright, I've played this for a few hours and this is what I think of the game so far.
Complaints
Drifting
Drifting is harder to do this time, and you have much less control over the vehicle while drifting compared to the last game. It kinda makes the drift challenges harder solely because of that, and because I can't keep the vehicle in place by holding the opposite direction like I could before. Now it seems the more you do that, the more it pushes you the side you don't want, but if you don't do anything you'll end up going to the other side anyways because that's the direction you are drifting in.
Control
This game is a lot more stiff and heavy in comparison to the last game and I can't say I enjoy that. I really enjoyed how the last game was more designed with control over your car than trying to emulate actual cars. The lack of control in comparison is a shame, but it isn't bad or terrible in this game, I just think the last one got that down pat pretty good so it feels like a step down.
AI is too difficult
Keep in mind before I say this that I've played practically every single Kart Racer that you can imagine and more so besides Crash Team Racing. What is cumbersome about the AI in this game is that they just don't let up no matter what, like they are kept in place because the game deems they should stay there. Kart Racers are known for this, but I think they need to tone it down with this one, it makes it hard to try to surpass the immovable first place AIs. I think for most people who are new to this game, this will be the key thing that may or may not break the game.
Item Balance
Items don't have the near perfect balance the last game had. Some weapons like the Firecracker sometimes just feel useless, but then there are items like the Hot Rod which is kind of ridiculous because it gives boost while sending your opponents flying and they lose all their speed with an AoE fire splash. Snowballs aren't as bad as the Hot Rod, but I think they're a bit much too. Now while I say this, I still think this game doesn't have things anywhere near as bad as Mario Kart does, however the last game was better in this regard where it seemed nearly all the items were useful in their own way, but not overpowered.
Options
The last game had it where you could turn off items if you were playing by yourself or for local co-op. This is a let down as well, and I hope the option to toggle them off will be added again.
Positives
Graphics
Graphics are quite a step up, I can see the shaders being put to use more compared to the last game. Metal Sonic looks divine.
Music
Music is fantastic, and the fact that they aren't just the original songs slapped into the game is a big improvement over the last one.
Stages
You've fixed the issue with the stage having the same themes as before and that's definitely refreshing to see as the last game had some rather bland looking levels because of it.
Etc
1. Good choice on picking the Metal Sonic song from Sonic 4 Episode II and Knuckles Sonic Adventure theme for All-Stars.
2. I do kinda like how it feels the characters have a bit more uniqueness this time
3. Think tricks were better in the last game, but I find it is nice they aren't freebies now.
Overall
I would be lying to you if I said I didn't think this is a good game, I certainly think it's fun to play, and it improved a lot of things from the last game, it just failed to improve the most important thing and that was the actual driving mechanic. That to me is a step down in my opinion, however it being a step down in that area doesn't make it a terrible or bad game. I still like this title more than Mario Kart 7, but they should remove the stiff/heavy/lack of control the cars have in comparison to the first entry.
The biggest complaint I see people having with this game is the AI, and although I may be able to put up with it as I am really into Kart Racers, I think it will shun people away who may be trying to get into the series. The AI puts me to shame sometimes, and that's saying something given my experience.
I suggest trying the demo before you purchase, but if you can put up with the AI then you'll have yourself a good game to play.