What are some fundamental rules for a map to look good?

Title.

There's a basic principle I've noticed that's really useful for getting nice visuals out of pretty simple geometry--I'm not exactly sure how to describe it but it could be called the "layered" approach; basically, whatever textures you're using, if you can create a sense of contiguous hierarchy between different materials, with "outer" and "inner" layers that are structurally consistent, it will register to the eye as pleasantly believable (believable, as opposed to realistic).

In this example (that I made in about two seconds) there's a central, brown, bunker-like structure, bordered by a concrete encasement on the outside, and more fragile electronics on the inside. There's a really natural progression of materials that feels intuitive and logical, and your brain is automatically filling in the blanks of what the rest of the structure must look like (if I tore away a chunk of that grey concrete, you could imagine there would be more of that brown/bronze material underneath). It's not so much about picking textures based on "realism" as it is about giving each material a specific role that feels consistent with itself and with the materials around it. If the textures were more mixed up with each other and not as neatly segregated, the overall geometry and form of the bunker would be a little more chaotic and difficult to parse visually. But here, each material that makes up the overall structure feels neatly organized and consistent.

It's not a super developed theory, but it's something I've noticed when making simpler, vanilla-style levels. Hope that makes sense to someone!

Some of John Romero rules on basic detailing and map design can work, you can look them up on youtube