Bubsy: Paws on Fire! has a really interesting level structure and I'm gonna talk about it.

The short version is that it allows for a lot of player freedom in approach and in forming the difficulty curve, and has a lot more content than it might first appear.

The long version follows.

The foundation is the four playable characters, each with their own abilities:

Bubsy the Bobcat plays similarly to what you’d expect from a Bit.Trip Runner game. His levels are 2D side-scrolling platformers. Bubsy runs automatically and can jump, ground slam, glide, and “pounce” (a forward dash-attack).
Virgil Reality (whom I had never heard of just a few days ago and who is now my spirit animal) plays very similarly to Bubsy, but with slightly different abilities. He runs automatically and can also jump and ground slam, but instead of a glide and pounce he has a double-jump and a “slide” for ducking under obstacles.
The Woolie is different. Her levels are also 2D side-scrollers, but she doesn’t run - she’s in a flying saucer, and while her levels scroll automatically just like Bubsy’s and Virgil’s, she can move freely within the current boundaries of the screen. Her only other ability is a forward-firing laser, and she moves more slowly while firing. (There are also power-ups she can pick up that give her a double shot or a triple spread shot.)
Arnold the Armadillo is even more different. His levels are like a 3D tunnel racer. The camera is positioned behind Arnold as he automatically rolls forward along a cylindrical tube, with his only ability being to move horizontally around the tunnel wall.

Every character uses their abilities to progress through levels, picking up collectibles and avoiding obstacles. There are 150 standard collectibles in each level which show an ideal path, but you don’t actually have to get them. They increase your score and can be spent on new costumes, but they have no effect on your progress through the game. The first time you reach the end of level with a given character (which is a significantly easier goal than getting all the collectibles) you get a “victory medallion” for that character and level. That’s the mechanism by which progress is gated - later levels require that you have some total number of victory medallions. But like Stars in Mario 64 or equivalent systems in other collectathon platformers, this number is always less than what you’ll earn if you finish each level with each character, giving you some freedom in what order you approach challenges and letting you skip ones that are giving you particular difficulty or that you don’t find enjoyable.

The one kind of weird thing is that Arnold is positioned differently than the side-scrolling characters, which has a couple of implications. Arnold levels aren’t immediately playable - to get access to an Arnold level, you must collect special collectibles as the other characters. Each of Bubsy, Virgil, and Woolie’s levels have three fragments of an Arnold medallion in them (sometimes hid more sneakily than the standard collectibles). Get all nine fragments and you unlock the corresponding Arnold level.

Arnold levels, in addition to playing quite differently, are actually bonus levels. You can’t fail them - the obstacles only break your point combo and make it harder to see what’s coming. And 40 of the 150 collectibles are put together in a giant unmissable collectible at the very end. So even if you have terrible aim or just put the controller down, you are guaranteed to hit the end and get a victory medallion and 40 standard collectibles. It’s legitimately a bonus reward for getting all the Arnold medal fragments and you can treat it as such if you don’t enjoy Arnold’s gameplay - but optionally you can also try to get all 150 of the collectibles and max out your score.

The downside of this structure is that the game looks shorter than it is. The game has three worlds, each with nine main levels and then a boss level. Each main level can be played as Bubsy, Virgil, or Woolie, and has an associated bonus level for Arnold. (Boss levels are one-shots.)

So this looks like the game has thirty levels, but in practice it has one hundred eleven - nearly four times as many, because each main level is really four levels in one. Officially you play the same level as Bubsy, Virgil, and Woolie, but their collectibles are placed differently and their abilities mean they take different paths. To me, they feel like completely different levels (especially Woolie) and if they hadn’t been grouped together on the map it wouldn’t have even occurred to me that they reuse content, because in practice they really don’t. They are three different levels - and Arnold’s of course are completely different since they are in a tunnel.

It’s a little unfortunate the confusion this framing can cause, and it makes the game look much shorter than it is which may put people off of the price tag (though it’s already at a budget price). I’m not convinced it was the best idea and perhaps there was a clearer way to handle it. But once you understand it and how much content it means there is, it’s a good thing.

So to recap, Bubsy: Paws On Fire!’s level structure means that:

There are three distinct styles of play (or four, depending how you count the similar Bubsy/Virgil styles - I find they feel different enough that I have a distinct preference for Virgil) making for frequent changes of pace. But you always know what play style you’re signing up for and won’t be suddenly surprised by having to learn a new style in the middle of a level or something. And you get to decide when to switch things up - I rotate through all four characters on each level before proceeding, but you could do a bunch of Woolie levels and then go back and do several Virgil levels if you wanted to.
Levels have a nice wide competence zone - the skill floor of just reaching the end of a level is all you need to progress in the game, and the skill ceiling of getting every collectible (without hitting any obstacles, if you’re truly hardcore) is significantly higher. This allows you to set your own goals and tailor your own difficulty curve. Since I’m an experienced Bit.Trip Runner player, I’m getting every collectible before moving on, but not requiring bonk-free runs - though note that the score combo system does grant higher scores to bonk-free runs, so if you want to show off your ability to do so on the leaderboards, you can!
Arnold levels are a legit bonus reward for getting the special medallion fragment collectibles, but also an optional source of additional gameplay and challenge if you enjoy them. I’ve seen some reviews call out these levels as their least favorite, but if you don’t like them you can basically skip them and still get a reward. I’ve grown to like all the play styles, and I actually prefer Arnold levels to Woolie ones.
There’s nearly four times as much content as there appears to be at first glance. I’m expecting the game to take me about ten hours, which is pretty good for the price tag. People coming in without having played Bit.Trip Runner might have more room to practice and learn the skills, and thus get even more entertainment out of the game.