The thing is, I've been having some trouble, even with help. Sometimes, I'll find paths that I don't think I've seen before, but I'm never certain. (As of 2011, I've spent ten years with Sonic 2.)
A friend suggested that I think of it this way: In Sonic 1, paths are more important than gimmicks, while in Sonic 2, gimmicks are more important than paths. (Gimmicks being elements specific to a zone: a gimmick would be a type of badnik or a zone-specific obstacle, such as the giant moving crates in Mystic Cave.)
Anyway, in Sonic 2, each zone has distinct gimmicks: Chemical Plant has speedy transport tubes and disappearing bridges, Casino Night has giant, elaborate pinball tables, and so on and so forth. However, within each zone, those gimmicks rarely create distinct passages. Each tube in Chemical Plant feels like the last, and each pinball table in Casino Night feels like I've seen it before.
(There's at least one exception I can name: the rising water segment of Chemical Plant Zone.)
For whatever reason, Sonic 2 doesn't get a whole lot of mileage out of its gimmicks. Maybe the gimmicks themselves are so simple that they don't provide a wide variety of uses. Or perhaps the level designers didn't want to use the gimmicks in a variety of ways.
So, after a single playthrough of any one zone in Sonic 2, I feel like I've seen everything the zone has to offer and I haven't seen the half of it.
On one hand, after a single playthrough of a zone, I've probably seen each gimmick used several times. On the other hand, I also probably can't name too many distinct scenarios created with them—and I certainly haven't seen every instance of the gimmick in the zone.
I want to say that this sensation is made possible by Sonic 2's lax sense of paths. Sonic 1 had very specific uses for each of its paths. For instance, in Green Hill Zone, the farther you strayed from the middle path, the more platformy the gameplay got.
Sonic 2 doesn't have that, instead creating more generic paths. You can freely flow from one place in the stage to another, and for the most part, it won't matter. In addition, because there are so many paths, you are less likely to memorize a certain act. Maybe you've been along one path, maybe you haven't. It's a lot more free-form than a single-path obstacle course.
(There's at least one exception I can name: the underwater paths of Aquatic Ruin are certainly different from those above the surface. On the other hand, the higher paths intertwine and change places so much in Aquatic Ruin that I think they are interchangeable in and of themselves.)
In the end, this makes Sonic 2 a much more relaxing experience than Sonic 1, and I think it accounts for why it's been my favorite of the Genesis titles.
Anyway—thoughts? I'm still very uncertain about all of this, so I'd appreciate hearing others' takes on Sonic 2.