I'm generally an easy person to please - make some interesting geometry for platforming, springing, and bouncing around, and I'm usually happy. Beyond that, I like it when I have to consider my situation and decide on what to do, instead of just executing the same action over and over again... which is to say, versatility of how a gimmick or badnik is applied is really good, in my eyes.
Above all else though, I love physics play. Stuff that screws around with your momentum - adds to it, reflects it, doubles or halves it, swings it around... stuff that interacts with your motion in a smooth, contextual way such that your motion from before matters and how you interact with it can have a huge range of resulting motion is really exciting for me. The old conveyor belt spin glitch, water surface boosting, bouncy floors and walls, gravity shenanigans, currents, Crawla Bouncing, and slopes are all good examples. Stuff that just replaces your momentum without regard for what it was before such as DSZ waterslides are not. Chain swings are kinda in the middle, since the ignore a lot of prior context about the player's momentum but still have a good bit of depth to them.
I like that kind of thing because there's a lot of granularity and depth to how you can interact with or use it - it gives you that feeling that maybe you didn't make it to where you were going for this time, but if you keep trying and tweak this part of how you entered and that part of how you traveled through the air, you'll manage it, and when you finally do it feels like you've just gotten away with something you really shouldn't have. Maybe you have actually gotten away with something, for that matter! But at its core, it's stuff that interacts with player physics in a way where it's fun just to screw around with, and which getting familiar with it can really help you out, and help you out more the more familiar you get. SRB2 really does not have enough of this kind of thing, even though it was such a core part of what made Sonic a unique platformer on the genesis.
TEI always stuck out to me because of its open, highly vertical levels where multiple paths were running above each other in the same room, which is cool in and of itself but which also led to some amazing crawla bouncing scenes. If this were expanded to include more physics play gimmicks while still having level architecture that lets you go really wild with it the way you can go really wild with crawla bouncing in some stages, I'd be super happy about that.