This is completely bizarre. The rules aren't creating these so-called gaps. I'll try to explain why. I'll have to de-construct your ridiculously long list to do so, so apologies if there are any formatting errors.
It doesn't need to. "DM decides" is an explicit part of the rules. Most of what you've posted here is a misunderstanding of the purpose of the rules. They don't tell you what fiction to establish. They provide a framework for adjudicating changes to the established fiction.
I put together your first and last paragraphs, because read together they're making me think we're talking past east other. I'll try to explain--maybe we can better isolate where we agree and disagree.
First, I entirely agree with you that "DM decides" is an explicit part of the rules. The vision/light/obscurement rules are short and non-comprehensive, by design. (And, as commentary, I think that's better than trying to be comprehensive, even if I would have written the short rules differently.) So the DM has to make a ruling in situations the text of the rules doesn't cover. The DM can't simply plop down light sources and objects/walls and have the rules exactly determine who sees what (as if the rules define the physics of light in D&D). Are we in agreement so far? I think we are, based on what you said above, and based on your mention in the discussion of the examples of the DM choosing light levels on a square-by-square basis--if we're not in agreement to this point, any clarification would be helpful.
The gaps I listed were examples of issues regarding light/vision/obscurement that the rules don't explicitly discuss. I apologize that they came across to you as bizarre, but since I think we agree that the DM needs to make rulings, I don't understand how the existence of gaps requiring rulings comes across as bizarre?
Where I think we disagree regards which of my examples are places the DM will need to decide, and which are covered by the rules. That's totally cool--there is room for disagreement over
where the rules are non-comprehensive without affecting my claim that the rules are not comprehensive.
You definitely seem to be taking issue with how literal some of my examples are, and I find that somewhat confusing because it seems to me like you are the one arguing in favor of a more-literal interpretation of the rules. For example, yes, I am familiar with what "opaque" means, and I would rule that anything opaque blocks someone from seeing beyond it. But it sounds like you would rule that dense foliage (which, absent magical transparent plants, is, of course, opaque) does not block vision of creatures on the other side. Indeed, both of the examples of heavy obscurement in the book other than darkness ("dense foliage" and "opaque fog"), are opaque, but it sounds like you would nevertheless let someone see past them unless "there is something opaque (like a wall or a tree trunk) in a heavily obscured area"? You'd expressed as much in response to
@Crimson Longinus's dense foliage example, which is why I included examples that didn't take the common definition of opaque for granted, since you didn't appear to be using it. Again, I apologize that you apparently found the example preposterous, I just don't see how it can be preposterous if you're (e.g.) letting the Dog see the Bunny on the opposite side of the dense foliage. Could you please clarify?
Are you serious? Of course! Again, you know what opaque means. You know how light interacts with opaque objects. You know what a shadow is. The rules don't have to answer these questions for you to be complete.
Similarly, my intent with the shadow example was not to raise something you would consider preposterous. The rules don't mention shadows with regards to illumination at all, and given how close a parsing of the text some posters in this thread have advocated, it seemed reasonable to me that someone (not necessarily you) would argue that the light from a light source would fill the entire area of its radius, spreading around corners, which in turn would affect the interaction of shadows and light levels. Does that make sense?
Since you objected to the length of my list of examples, I'm going to not discuss the others--I think we can probably nail down where we are disagreeing with what we have so far. I appreciate that you took the time to respond to all of them, and if you would instead prefer a complete response, please let me know.
