Gygax said a long time ago, that the secret to RPG's was that the players didn't actually need the rules.
My takeaway from what Mearls is getting at, is that the rules are less important than a coherent framework for us to base our games around. Rules provide an easy resolution system for the creation of our shared fiction, but, if their relation to said fiction doesn't make intuitive sense to us, then it causes dissonance.
For instance, the longsword in game does d8 damage while the dagger does d4. This is not only a valuable game construct, but is intuitively graspable by anyone who sits down at the game, because the longsword is bigger than the dagger, and we would expect the longsword to be a more dangerous weapon. By contrast, a warlord allowing for the recovery of hit points is a harder conceptual leap, because there's nothing intuitive to someone we imagine as a "warlord" having healing abilities.
Now, once we conceptualize hit points as fatigue and the warlord as motivating us to overcome our own limitations, driving us to succeed when we'd rather lay down and die, the warlord makes sense, but it's not an easy thing to grasp and it takes a while to internalize, whereas hit points as wounds and hit point recovery as "healing" is intuitive, even if the extrapolation leads to some strange places.