Good question.
My favourite video game genre is platformers, by far.
I tend to prefer 2D platformers to 3D because of the physics. Good 2D platformers have a very good feel to them—you have tight control, fast running speed, good aerial maneuverability, and your jumps actually feel like a consequence of your previous movement. Running and jumping are not two separate things. I could replay Super Mario Bros. 3 every day and not get bored because the game feels good to play.
Compare that to most 3D platformers (think 3D Mario). Your character is very slow by comparison, jumping halts your momentum and follows a noticeable arc as if you're playing a grid-based game, and the controls just generally hold your hand a lot more because of the inherent difficulty in platforming a 3D space (the Sonic Adventure games are not like this, but they compensate for the 3D difficulty by making the controls hypersensitive and letting you ignore momentum, which is its own problem). With that said, I still like 3D platformers because I like the feeling of freedom and exploration they provide. I couldn't replay a 3D Mario game over and over like I could a 2D Mario, but I do very much enjoy the game the first time I play it.
Enter SRB2. It—remarkably—has ported over the feeling of a 2D platformer's controls over to a 3D game (at the expense of a ridiculously high learning curve). The best of both worlds. I can do precise platforming at both low speeds and high speeds; running and jumping feel natural, both feeling apart of and affecting the other; and it just feels so fun and not at all like any other 3D platformer I've ever known. It's so good. Like Super Mario Bros. 3, I could sit down in front of the computer and replay SRB2's single-player campaign for the fortieth time and relax and have fun, because the controls and physics just feel so natural and responsive and freeing. I could play an utterly crappy level of SRB2 and still have fun because I love the feeling of just simple running and jumping so much.
I really do love this game. It's probably partially a fluke that it turned out this way, but that's not important. I don't know if anyone else feels the same way I do, but I like to think that there's at least some aspect of it in play because I don't see how else a fangame could still be popular after fifteen years.
