Each zone also has unique enemies :V
Yeah, for better or worse that makes Sonic what it is. You don't get as many individual levels as in, like, a Mario game, but no levels will dare show you the same thing twice. It creates a unique sense of progression throughout the game. It's a cool alternative to the level trope-based norm.
Yeah, I'm impressed by the sheer scope and scale of each level, and how densely packed they are with new ideas — unique enemies, obstacles, bosses, stage elements, music, environmental art — and how you have to replay each level multiple times to see every path, and how each path takes a decent amount of time to explore despite the fact you're (potentially) moving fast, and how you can get additional replay value by going through them with different characters or pairs of characters, which sometimes even modifies the layout or straight-up changes the entire level (Knuckles' Mirage Saloon Act 1).
I also like how many platforming sections have the payoff of a pinball section where you're rocketed around the stage (with varying levels of control), which may be 90% flash but is still exhilarating and enhances the sense of scale, since you're often weaving past previous sections of the stage but arriving in new areas, like when a loop-de-loop travels behind the foreground, or when you run along the other side of a double-helix bridge, making everything feel layered and interconnected.
The level design is very impressive. I'd be interested to see a map of an entire level, if such a thing is even possible, if only to see how the big picture fits together. Some stages seem overwhelming in their complexity, to the point I wonder how they even went about designing them.
