I'm not talking about the physics or controls or level design (well, a little about the latter), but in the way 2D Sonic uses multiple characters in a way basically no other platformer off the top of my head does. Because really, think of all the platformers you know, 2D and 3D, and now think of which ones use this specific structure: -more than one playable character -each of these character is locked to a savefile and you cannot switch -multiple routes in each level for each character--that is, the differences between the characters aren't just their movesets but what content they experience at all -each character is treated as equal to one another by the game--beating S3&K with Tails is 100% valid and as respected by the game designers as beating it as Sonic, that is; Tails goes through all the same stages, he has the same goals, you get to see the credit and collect all the emeralds with him, his levels aren't made easier, etc. etc. All 3 characters (5 in Mania) are considered valid and there's no "correct" character to play, with the others just being considered bonus content ...because I cannot think of really any. Like, platformers don't usually have multiple playable characters at all, and when they do, you either are expected to swap between your crew to solve challenges (i.e each character isn't a full on totally independent character, each one just can't do certain things, and beating levels with only one often requires glitches/exploits to bypass stuff they're barred by the game design itself from overcoming--DK64 is a prime example of this kind, as are the Sly Cooper games I think?), or there's one "correct" character and the others are just fun bonuses with often weird abilities and who can't go through the main campaign (Super Meat Boy is a very good example). Or they're just identical skins for multiplayer and replays the like. The thing that makes 2D Sonic so distinct is that each character is equal, and beating the game as any one of them counts as beating it. Like sure, Tails is easier, yet, but the game doesn't go out of its way to restrict him. Knuckles does have the restriction of not being able to jump as high, but that's just a unique trait and his routes aren't weird gimmicky bonus content, they're full-fledged alternate pathways with the same care as the rest of the routes. Ironically enough (given that most of his games have one playable character ever) Mario has some examples that almost get all the way, like SMB2 USA and 3D World, but there's no alternate pathways and the characters are barely differentiated, and you aren't locked to one in a savefile. People always talk about the physics and level design as what sets 2D Sonic apart, and yeah, that's true, but I think this is another aspect of it that basically no other platformer series besides ones that actively tout it as inspiration like Freedom Planet does. And before someone points it out: yes, I know that 1, 2, and CD don't follow this formula and 3&K started it. It doesn't matter because the overwhelmingly majority of 2D Sonic fans consider 3&K to be the pinnacle of 2D Sonic design-wise and the Advance series, Rush games, Mania, and the huge amount of fangames that aren't romhacks of 1 and 2 (and hell, some of those still have multiple characters!) all follow this model of multiple equally-important playable characters.