I cannot speak for others, but for me, when I was a kid, what amused me the most in the game was the speed and the "weight" of Sonic. The way you gain speed when you learn how to use the terrain, the inertia of the game, I think. After you play each stage several times, you can complete them in absurd speeds:
(Oh, and the game was really beautiful. I mean, Super Mario World looks nice, I guess? But at that time, the amount of detail, color and everything else blew my mind.)
The gameplay is just fun, no other way to say it. But here we end up walking into the old question of the "subjective nature of the taste", and we all know where this is going.