What about "harnessing" your ability - or the potential of the ability.
That's the idea, I think, not represented well by the idea of "training" the ability.
Yeah, I think I understand where he (they) were going with this. The concept is sound, but I think the mechanice already exists - Ability training increases the Ability Score. Whether through focused or generalized training, or just accumulated experience, one increases their Ability Score (in other words: standard Ability Increases). Alternatively one could make Feats for Ability Training rather than set Ability Score increases.
I think the "potential" of an ability is already quantified by the Ability Score (or more specifically, the bonus for the specified Ability Score). Adding ranks to it is just an extra layer of complication that to me, doesn't add anything and is less realistic. Want to change the potential of an Ability, then change the Ability Score. Seems much simpler to me and still accomplishes the same thing.
I think the main thing driving Mearls concept though, is trying to find a mechanical way to make Ability Checks comparable to or unified with Skill Checks (because Skill Checks have skill ranks, etc., and Ability Checks don't). I think that can be greatly mitigated just by adding character level to the check. There's still a bit of a disparity, but that works for me. Were talking about the difference between using untrained raw talent vs. trained specialized skills for accomplishing a task. Specialized skills will win every time for a specifically related task.
On what do you base this? Could it be that your perception of the world comes from game concepts that you have internalised?
My talks with people who have read sports science suggests that initial training quickly allows people to correctly harness their innate ability (STR,DEX, ...). In game/maths terms you go from one side of the bell curve to the other for standard task (so say - 25% to 75% of success). The very game-y thing of having 50% chance of doing something is not how learning something works. You switch from can't to can.
In a related area however, e.g. chess ratings, if two equally skilled people go head-to-head then it's 50/50 on who wins. That's sort of obvious when you think of it but a handy thing to absorb.
Yeah, we agree on opposed checks. But not on the rest.
For instance, if one does intense training to improve one's general Dexterity - even to the point of achieving Grand Mastery level (Mearls words

) with it - that still doesn't make him able to go head to head with Micheal Jordan (who would have Grand Mastery level with Athletics: Basketball), no matter how high he pushes up his general Dexterity Skills. Likewise, somebody doing "Grand Master" level Strength training, is still not going to outswim Micheal Phelps (Grand Master at Athletics: Swim), out drive Tiger Woods (Grand Master at Athletics: Golf - or used to be at least

), or out hit Derek Jeter (Grand Master at Athletics: Baseball). Technique is what makes them different from the generalist (and a big part of that is muscle memory).
With Mearls concept, the "Grand Master" Strength or Dexterity generalist would be just as good as the specialist at their specialized skill - and that just doesn't transfer to the real world. By that logic, an All-Pro NFL Wide Reciever should be just as skilled at Basketball as an All-Star NBA Forward - and vice versa - and that just ain't true. The Wide Reciever would be better at basketball than Joe Blow off the street, but would probably get smoked by the NBA player.
However, I can see the logic behind providing a synergy bonus to an applicable Ability check if one is trained in an Athletic skill. So, along with adding level to an Ability check, add a synergy bonus of +2 for each "level" of expertise in the highest trained applicable Athletic skill. For instance, if they are a Grandmaster at Swimming (5 levels: Novice, Journeyman, Expert, Master, and Grand Master) then they'd have a +10 synergy bonus to Strength checks along with a bonus equal to their character level - and still use a unified DC system for Ability checks and Skill checks.
I'll freely admit though, I come at mechanics from more of a simulationist approach than gamist approach. I'm not saying that your approach is gamist - I don't know what your preferred style is - and even if it is gamist, that's cool. I don't have a problem with other styles. My preference however is simulationist, and Mearls idea as presented just doesn't work for me.
