So how are you judging the "average" person?
Every author who writes a book that is intended to teach (amongst whatever other intentions the author may have) must answer this question. Having never written a book of any kind, I cannot give you a productive answer. I lack the experience or training to confidently and comprehensively answer. Seeing as how there are many guides published every year, it seems quite clear that the question has at least one answer (and probably several.)
However, in general, based on what training I have with pedagogy (I am
not a teacher, but I have taken some classes in education), I would expect that one would need to collect data on the kinds of questions which come up frequently, the issues that experienced people highlight as problem points they've encountered and wish they had understood sooner, and important baseline information needed for communicating with experienced users. From there, testing and evaluation: have people read it, then practice with them and see what comes out. Iterate at least a couple times.
Does that seem unreasonable?
And what exactly do the 3 corebooks leave our that is essential to playing D&D 5e?
Examples are a major thing. Examples of how the game's content can be adapted to suit distinct ends (e.g. curating races, classes/subclasses, spells, feats, etc. to produce a desired theme or tone.) Examples of how to adjudicate difficult situations and the ways that individual preference and judgment calls can positively and negatively affect the play experience. Examples of how to improve consistency, and how to take effective notes. Examples of how to resolve interpersonal conflicts, build consensus, and acquire and preserve player trust. Examples of the contrasting virtues of effective DMs: flexibility vs firmness, consistency vs context sensitivity,
why you should sometimes ignore the dice when you don't like what they say and sometimes need to follow them even when you don't really want to. Discussions of alternate perspectives on how something can be handled or considered, akin to the 13th Age core book with its running developer commentary (which can be incredibly useful!)
Equally important would be tools that actually work reliably and effectively across a broad spectrum of situations. 5e CR is crap and even 5e boosters usually admit it. Consequently the encounter building tools are largely useless, and often
worse than just "eyeballing it." Given the importance placed on combat, that's a pretty serious oversight. I'd also like to see rules and advice for magic items (finding, seeking out, buying/selling, crafting, etc.) The extant ones are minimal at best, not exactly high quality, and (IIRC) spread across multiple books. I know that much of this is due to the designers having a massive overreaction to the importance of items in 3e and 4e, but that doesn't make the current situation any better. Finally, a widely cited frustration with 5e is that it falls down in exploration content, despite that explicitly being a "pillar" of the game allegedly as important as social or combat situations. The DMG clearly isn't up to snuff on teaching DMs how to do that well within the context of 5e rules.
