I forgot...Half of me agrees with you, but the other half says, "If you want inspiration writing, their are a hundred non-system-specific GM Help books." Due to that, I don't see them going too deep into the fluff, unless it pertains to specific setting information.
I appreciate the light touch-dm empowering approach of the PHB, but some sample DCs, equipment/materials stats, more on non-combat situations, etc could be handy.
Speaking of which, DCs as presented under Ability Checks (p.174) seem to be a bit misleading for a novice DM particularly, or maybe I'm missing something? The table gives the following values for DCs: very easy (5), easy (10), medium (15), hard (20), very hard (25), nearly impossible (30).
Under these guidelines, an average, unremarkable character (say, 1st level, no ability bonus, but proficient with a skill) would have 35% chance to FAIL at a task that's purportedly "easy" and 60% chance to fail at a "medium" task. Similarly, a 20th level character with the best relevant ability modifier (+5) and proficient with the relevant skill (+6) would still have a not insignificant chance (20%) to fail at a task that is only of average (medium) difficulty. Doesn't that sound a bit suspicious where math is involved?
Now, it's quite possible that I'm looking at these numbers in isolation, and that are other ways of boosting the chances, e.g. giving help, advantage, etc, but still... Maybe simply to change the wording a bit? Lose the "very easy" DC because, really what's the point of that; simply assume an automatic pass for a trivial task and leave it at that. Bring all other DCs one category lower - easy (DC 5), medium (DC 10), hard (DC 15), very hard (DC 20), etc. This way a 1st level character has solid, but not too great, chances of making medium DCs (as it should be in my opinion) while guys who are very high level, proficient, and with high ability scores have a somewhat easier times meeting high DCs.
Right now, I see 15 as the challenging default. That is, something looks kind of tough...DC 15.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.