So most people are probably familiar with how Mario levels usually work, right? Introduce a mechanic and iterate on it, hit the flagpole, onto the next one. This concept has served many games well over the years, though you had better make sure that each mechanic is sufficiently unique and memorable, and to build a detailed and satisfying level around it. It's the difference between getting NSMBWii, the shortest NSMB game, in which many levels still just fall back on "I guess we'll make the ground spin like a wheel" and have huge empty spaces to accommodate for multiplayer, and NSMBU, which operates under the same restrictions yet flourishes. The Donkey Kong Country series works much the same, it's a popular format, one might say it's the basic building blocks of a platformer in general. Alternatives certainly exist, but have mostly fallen out of favor. It's hard to think of a popular 21st century platformer that isn't designed like Mario in this respect, it's such a solid fundamental process that it's just as applicable to something like Zelda even, and has been used in that series as well since Wind Waker.
In Galaxy 2 though, levels have subtly moved away from being built like your standard Mario stage. Galaxy 2 levels are not so much based around a mechanic anymore, but a theme or concept. It's a subtle difference, most of the time, but it can result in levels like Slimy Spring Galaxy, which feels way more at home in Donkey Kong Country than Mario. This was sort of present in the first one, but the levels tended to be a lot less coherent and focused, with it sometimes being obvious that they made some planets and threw them into whatever galaxy had room (Gusty Garden's Gravity Scramble comes to mind). Galaxy 2 is by far the most DKC-like Mario game. The power-ups work as alternate gameplay styles to build a level around like the animal buddies, and setting has become much more of a defining factor for each level. DKC, while it often worked similarly to Mario in practice, didn't always have to. Squawks's Shaft and Rambi Rumble are levels with completely distinct first and second halves, for instance. Not too weird for Galaxy, but past (and future) Mario games wouldn't really do this because each level is meant to be a short, self-contained idea. And mind, neither DKC nor Galaxy 2 are really fundamentally breaking away from how Mario levels are designed, it's more that they're just moving beyond the most basic expression of that concept where the loop of "introduce mechanic > develop mechanic > move on" has to be what defines a level's macro structure.
Take Squizzard's Sandy Sinkhole. The level is about moving sand and fireballs, with a side of crushing hazards. But these ideas are not confined to a singular mechanic. First you have a planet with sand streams that push you along, lit torches, and rings of fireballs to dodge. Next planet, the sand moves against you, which is dangerous because there are enemies everywhere and blocks that slam together to crush you between them. You can get a Fire Flower to throw fireballs and clear out some of the enemies or light a couple torches for a 1-Up. At the climax, it turns more vertical and you need to climb on top of some of the moving blocks to reach higher ground, forcing you to interact with the hazard in a new way. Third planet is a sand slide with Rhomps rolling down to crush you, then it's putting pressure on you to stay centered as it gets narrower and narrower towards the bottom. Lastly you're onto the boss, who is in a pit with sand sliding down towards him and must be defeated by throwing fireballs into his mouth. It allows for levels to be overstuffed with ideas and constantly change up what you're doing, yet it still feels coherent because there's a central concept tying it all together. Each planet here kind of speedruns the basic progression of a Mario level, but without really losing anything in the process, because it's a simple progression that can fit into levels as brief as Mario 3's. First you have a ring of fireballs on solid ground, then on moving sand, then one spinning opposite the last, then two close together going opposite directions, then a whole sequence of those on the sand to cap it off. They didn't cut out all the other playful stuff that makes a Mario level, nor is it especially difficult since they don't push or explore the mechanics any further than they normally would, they just had more confidence, wasted less time. It's what makes Galaxy 2 hands down my favorite Mario game, and I'm doubtful there will ever be one with this kind of confidence again.
While Returns was similar to past DKC games, Tropical Freeze would take this upped variety and run with it. A level would be about alp horns and falling leaves, or a wildfire, or being attacked by a fruit-picking mech, and it would come up with a bunch of different mechanics within that concept. But rather than going from one thing to another, it tends to keep mechanics around for the whole level, combine them, and strive to utilize each one to the fullest extent. Windmill Hills starts out interspersing windmills and rickety platforms, but at the end of the level, there are windmills made of rickety platforms, which combines the ideas and creates a new twist and culmination of difficulty for both of them at the same time. Now the windmill platforms break, and now the rickety platforms are spinning! The level though is otherwise way too long and complicated to describe in the same level of detail I did Squizzard. Even ignoring the various little interruptions and "minigame" breather segments, which are particularly plentiful in this level, there are a lot more instances of each hazard's evolution to discuss when they aren't confined to a singular segment, and going through an overview of one segment at a time doesn't really work because they don't divide as cleanly. It's definitely doing more. With the slower pacing of Galaxy, Tropical Freeze levels don't end up ridiculously longer on average I think, but a slower-paced game that goes for everything Tropical Freeze does can become overwhelming.
