The only problem with massively improving the game in every aspect, including visual design, is that now the visual design is good enough that it's worth critiquing!
This, of course, applies for level design, too.
In the eyes of someone fairly active in the Discord community, it seems like this topic has been on everyone's minds for a while, so I'll leave the majority to speak here. But explaining the crux of it for the unaware, Techno Hill isn't really interesting, is it? While THZ1 has some semblance of interesting design (key: some), I'd argue THZ2 is the worst level in the entire campaign.
And then, there's the goop. While a good gimmick on paper, you can feel the remnants of pain water in the level design. In THZ1, entering the goop is always a hindrance due to its placement being in areas where you'd usually fall after failing platforming -- in act 2, you're forced to go through it once, but if you're a skilled player (or you're playing as Tails) that's the only time you'll actually come into contact with it. I'd say even Haunted Heights uses goop in more interesting ways.
I know this is a topic fluctuating on a lot of people's minds so I wanted to hear whatever y'all have to say anyway.