Like a weird mix between premature birth and a really frantic brainstorming session. Sonic 1 feels half baked and unfinished, it's nearly there, but not enough even as a first attempt.
It unfortunately creates a game that isn't that fun to play most of the time, even when you learn to play it "properly".
But you can feel they had something here, I think they just didn't really know what it was yet. They needed some time for their baby to grow.
Music is great tho.

(making Scrap Brain 3 just Labyrinth Zone 4 is so nefarious and sadistic it's genius, it's like the perfect in-character trap for Eggman to set)