For me, taking it all in after 100%, it's a great game but it will always be held back in my eyes for how they made Mario so damn sluggish. Go back and play some older games and see how much faster he runs, how much easier it is to change directions, how much farther your jumps go, how the wall jump physics are less rigid (jumping at angles other than straight away from a wall), and just generally how responsive Mario feels.
There is one reason and one reason alone (broadly speaking) that the platforming feels fun in this game, and that's Cappy. I mean first of all because you get to platform as creatures other than Mario, some of which are great. But also in the sense that Cappy gives you a ton of movement options. He lets you shift directions midair and do cool bounce sequences that give you immense air and/or distance. It makes you feel really powerful to skip entire chunks of platforming just by doing say: ground pound -> bounce jump -> Cappy throw -> dive onto Cappy -> wall jump -> second Cappy throw to reverse direction -> Dive onto a ledge. Without Cappy though, it's kind of terrible.
Now there are still platforming sections that make the worst of the game's physics, but the segments that take Cappy away, especially in post-game challenges, really make it feel like a 3rd-rate platformer. I'm restating what others have already said (including me), but it bears repeating in this retrospective on the completed experience. It just feels bad. Your ability to move in a way that's anywhere near enjoyable is gone. And it's compounded by what I feel is poor hitbox detection/implementation. It's really frustrating when I expect to grab and climb a ledge but Mario decides to wall jump half the time, or when a roll or long jump really looks like it's clearing an obstacle but a one-pixel edge of the hitbox makes me crash, or when a spin jump fails to execute because who knows what it thinks I inputted. Why not add to that the terrible decision of making the grab action (for rocks etc.) the same button as throwing Cappy (thus flinging them into oblivion), or simply the hitbox not wanting to activate so I try to get closer and kick them away. Or I don't know if this has happened to anyone else, but jumping on an enemy but the game deciding you didn't really land ON an enemy, in a way that will kill them, but you did land IN the enemy so you're taking damage.
The above wouldn't be so bad if the game didn't give you these post-game challenges that require a lot of precision, when everything feels muddy and imprecise, from Mario's handling to the hitbox issues. Because yeah, I like a challenge. I was waiting for a long time for the game to get difficult. But in many cases I didn't feel the difficulty came from overcoming challenging scenarios so much as overcoming poor design. I did enjoy the final unlockable level overall, because fortunately it mostly avoided such scenarios. Mostly.
Oh also the fact that there are no ways to damage capturable enemies as Mario other than jumping on them has this rather weird and telling consequence of Goombas being practically the most terrifying enemy in the game. You can't just kick or spin attack them or repel them with FLUDD.
So anyway like I said the game is a great experience, but I think that's only the case because it's pretty easy. The best challenges are the transformation-based ones. Design-wise there is some truly great stuff in there, and a few areas/references alone elevate the experience. But it's also lacking in areas such as music. Very little of it is memorable to me. Wooded Kindgom stands out in particular to me though. A lot of the rest is enjoyable enough but just seriously not memorable.