"tight" as in tightly rendered: clear and well tuned. Not a criticism at all -- 'Warning' was one of the tiles I was complimenting

My perspective choice on the vertical pipe probably -was- off (partly because I tried to extend the suggestion of creases that you had already partly defined, rather than working out what perspective would work best). But you could show structures that have a range of planes ( in both visible axes, not only one like the old pipes)
Anyway, the perspective issue may be unimportant if you are constrained to such a small number of tiles.
I was really thinking about things like showing some of the top surfaces of platforms and having the player walk along the middle of those; to me this would already make it feel much more solid.
BTW that is a -very- stringent limitation, it makes me wonder why.
Hah, I didn't realize it was supposed to be corrugated, I just thought "how can I disguise the dark/light transition? ... Rust."
Crits of updated image:
* Stray yellow pixel in BR of warning-striped pillar.
* Much prefer the new acid/whatever-it-is, far easier to look at
* Cute droplet. Is it supposed to fit in an 8x8 tile or not??
* The warning-stripes variation/shadow really help with the 3dness

* Perhaps you're being a bit too realist with the secondary light. Within reason it's just there to help you make nicer clusters. If it doesn't (eg. wires) you can change the basic 3d shape you're applying it to, stylistically distort clusters, or just leave it out altogether.
* Consider being more sparing with the purple in the BG, swap it out for some gray maybe? too much saturated color makes it read more foregroundily than you probably intend.
Have you done any research on EGA graphics? for example you can see a broadly similar style to what you are using here, in
Secret Agent. There are a few things to note there, for example the heavy -- perhaps a little too heavy -- use of patterns and high structural detail to break up monotony, both on background and foreground elements.