On that point, I think Mearls may need to consider that it's about as much what he leaves out of the recipe, as what he puts into it.
It's a good point, that the presence of these elements doesn't 'override' all else in order to ensure a game is D&D. But, again, that isn't really what Mearls is looking at.
Mearls never says, "Anything with these elements is D&D". He is explicitly trying to find elements that, if they are
missing, make a game feel less like D&D, and/or elements that, when they are
present in other games, make you think of D&D.
Likewise, identifying the core components of a "universal D&D" isn't enough. Consider what may un-D&D a game to a large segment of players, such as through:
1) Disassociated mechanics like healing surges that break simulation and suspension of disbelief. I gather it is rumoured that healing surges are Mearls' favorite 4E rule, so may be in for inevitable disappointment there.
The problem is, at this point we're getting into much more subjective areas. For me, Healing Surges work fine... because they build on the already simulation-breaking coincept of hitpoints, which took me
years to come to terms with. Those elements have always been abstracted (I hate the term 'disassociated', which has largely lost any real meaning in these discussions).
I think it is also an area where many of the fundamental objections are not what is actually put forward. Healing Surges as a concept is really just a new way of formatting hitpoints. Connecting them to magical healing might jar some folks because it is a new method, but is certainly not anti-simulationism. Instead, I think the real objection is the fact that all healing surges are restored each day - characters 'healing to full' with one night's rest.
And that might be an issue worth debating - but it also isn't tied to healing surges at all. It would be just as jarring, for the same folks, if you healed to full in 2nd Edition, without only hitpoints in play.
2) Including in the core mundane yet fantastic equipment like gluebags, absurd dual weapons and absurdly effective spiked chains that harm D&D's ability to model a pseudomedieval setting. Save it for the splatbooks, IMO.
3) Quirky and specific choice of core races and classes in the core PHB, and the resulting specific nature of the worlds they suggest, as I've already talked about.
This, again, is getting back to the argument about 'core elements'. D&D isn't, at its heart, about any one specific world. If it was, campaign settings just wouldn't exist. There may be a specific default setting you prefer - such as low fantasy - but there is a big difference between claiming it as a preference, and insisting everyone else who likes a different setting simply isn't playing D&D.
I like Hackmaster, but Hackmaster isn't an "everyman fantasy" game which is a good basis to build worlds on in the same way D&D used to be. Not every world I want to make has these specific elements that Hackmaster pushes. 4E has made that same mistake, IMO - in attempting to stay current with fantasy fashion, they've traded in the game's utility as a fantasy toolkit. If Mearls is casting around for ideas as to supplements for 5E, maybe add a "High Fantasy Handbook" and a "Dark Fantasy Handbook" for those who want to take the game in different directions. That way, the dragonborn spiked chain wielders and gluebags can sit in the high fantasy book, and whatever low magic people want can sit in the other.
I'm not sure it really counts as undermining the 'fantasy toolkit' just because the default has more high fantasy elements - after all, you can always take them out and run a low fantasy campaign, just as you can do the reverse in a game that starts from a low fantasy default.
Still, I do get your point about presenting both as options. And, apparently, so does WotC - have you checked out Essentials? We've got two books of player's options. Book #1 gives us humans, dwarves and elves, along with the cleric, fighter, thief and wizard. Book #2 gives us tieflings, dragonborn, and drow, and the warlock, druid, ranger and paladin.
One for the classics, one for the more fantastic elements, making it relatively easy for a DM to tell players to only go with one or the other. Though I don't think the spiked chain got specifically targeted as 'high fantasy' for that purpose... and, honestly, I can't find it any more absurd myself than many of the other weapons D&D has seen over the years.
One of the reason why I think some of the 'flavor elements' can't really define D&D - they are far too tied to personal preference.