So you think Mearls is wrong because the ambiguous rules don't "let" the DM decide, they "make" him decide. Whereas clearer writing would "let" the DM decide. Is that right?
Maybe.
I think clearer, simpler wording that said "you need to be concealed, and if there's doubt make a Perception check (passive or active as appropriate to the perception rules)" would make it clear that fictional positioning is key. And the norm in D&D is that the GM resolves disputes over fictional positioning. (This wording would also allow modular variation, though - eg a group could use "plot points" that let a player declare the fictional positioning for a particular hiding effort.)
Whereas a whole lot of technical wording that is ambiguous I think creates a misleading impression of a bit of a mess that the GM has to sort out.
Precisely. If 5e just wants the GM to adjudicate if Stealth is legitimate and then roll something simple, then why the rules language that canvasses the intersection of multiple mechanical vectors (concealment/cover, line of sight/effect, passive perception, etc)? Presumably, with such engineering there is supposed to be a process of following a "if yes then < >, if no then < >, and so on" flow-chart in there somewhere.
If those rules intersections are irrelevant, if the engineering isn't about the adjudicative process following an if/then flow-chart, if in the end its just going to be a throwing up of the hands and "bloody hell...just figure it out yourself", then why all the "stealth stuff?" If we're looking for a lean, mechanical machine that is "fiction first", intuitive, puts the keys to the engine in the hands of GM discretion, and requires minimal handling time at the table, why isn't 5e looking toward something super simple like Dungeon World's basic resolution mechanic:
1) GM relays fictional positioning.
2) Player makes action declaration for a Stealth move.
3) GM decides if the fiction warrants something else happening before Steath can be made such as:
"These guards are on high alert and the passage you're transiting is fairly well lit and fairly low on stuff to sneak behind and obscure line of sight. You're going to need to figure out the patrol routes/frequency or come up with some kind of misdirection or something (eg Discern Realities or Defy Danger - Int - for the misdirection)".
4) Player can then make their action declaration for DR or DD (or something else if they've got a better idea and it fits).
5) Resolve that and lets find out what happens.
6) If the fiction that comes out of that dictates that things are still a go for Stealth, then have it. Resolve that move and lets find out what happens.
Done and done. All of these (a) intersecting, granular Stealth rules when (b) the designers say they've aimed towards low mental overhead and quick table handling time (and hopefully *
intuitive action declaration for players such that they have requisite agency) while play is ultimately going to turn on (c) "forget all of these intersecting Stealth rules...GMs, its on you to make sense of things"? It doesn't make a whole lot of sense. Yes, I know it
feels very AD&D and I guess that is good for nostalgia. Still doesn't make a whole lot of sense.
* Incoherent or unintutive rules, or rulings that must be made to manage their inadequacy or silence, are quite often prohibitive to intuitive action declarations by players.