Steel decoration: steel or earth?

[Proxima]

Your eyes make the determination based on other surrounding pixels forming a pattern consistent with a given set of terrain pieces, some of which are designated as steel while others are earth.  And even to do that, you would have needed the player to have known beforehand which terrain pieces are steel vs earth, either via the editor or via examining the actual files defining the terrain pieces in question.  Otherwise they would still be falling back to either intuition ("gasp!"), or past experience based on testing out the effects of terrain removal against some particular terrain, to determine if something is steel vs earth.

For example, the 32-wide metallic grey pillars in the concrete set are not steel. The 8-wide pillar that looks very much like a shrunk version of the same tile? Steel.

[Nepster]

Here another completely different argument: We already had in vanilla Lemmix the possibility to hide steel (by placing steel areas over usual terrain). Sometimes this was horribly abused: Try to guess where the steel areas are in levels 1, 4, 5 and 9 of CHRISFN15.dat (Yes, they all have steel!).
While I have faith in all forum regulars to use the ability to cover steel with decoration properly, I wonder whether new Lix users will feel the same? The Lemmix history suggests something different: If some option can be abused, than someone will abuse it.

<geoo> Proposition for the steel issue: a class of decoration (ideally with alpha) that does not affect what's underneath, and if placed on air will just be erased. I cannot be used for any other purpose (e.g. terrain), thus it's only in the tileset designers hand to provide visually distinctive decoration
[...]
<geoo> and it can also be applied to normal terrain, but not to air
<SimonNa> we still introduce an extra type of tile. I'm not 100 % convinced that bombing holes into steel is such a problem
[...]
<SimonNa> deco is gadget that is never checked for interaction
<SimonNa> leftover from "let's make lots of crap like trampolines"

If you go with this, I would suggest implementing the current deco objects similarly to what NeoLemmix calls "objects with Only-On-Terrain setting": This is a class of objects that does nothing except recoloring already existing solid terrain pieces.
Advantages:
- Removes unintuitive deco pieces that look solid, but are not.
- Allows steel decoration pieces as this new object type.

[ccexplore]

If some option can be abused, than someone will abuse it.

That may be, but there are plenty ways to create bad levels far beyond the scope of this topic; if someone's that wary of encountering one, just stick to playing levelsets and/or particular level authors that others have already played first and provided public feedback on, or stick with well-curated sets like the community set.

And let's not pretend that earth cutting into steel isn't opened to abuse.  It's not that hard to cut a hard-to-see tiny grayish earth crack into a steel block.

[namida]

I've mentioned that point multiple times in IRC - it's simply not possible to enforce every possible guideline as to good level design. So, if enforcing one would have a negative impact on level design potential (especially in terms of physics, but to a good extent this holds true even for aesthetics), it's better to not enforce it and rely on a guideline instead.

Perhaps what then needs to be considered here is a way to encourage newcomers to read the guidelines (or otherwise realise them). Yes, you'll inevitably get some that don't, but they'll just have to learn the hard way when no one likes their levels. Perhaps a "Examples Of What Not To Do" pack included by default that's as annoying as possible.