Just a few things.
I really, really like to hear Mearls think out loud, but was very disappointed by the indication that these musings weren't going somewhere. For a while, I was excited by the thought of a modular, dial-in complexity as desired, and was hoping 5e was being kicked around, design-wise.
Second, following from the above, I was hoping for a basic, underlying truly D&D system for 5e. That said, did anyone else notice the complexity that Mearls seems to assume or take for granted?
"When you are climbing, all attacks against you gain combat advantage. If you cannot take standard actions while climbing, you immediately fall. You also fall if any effect forces you to move against your will or if you are knocked prone."
For instance, if this came from a new edition, I'd be doing the following: looking up what the heck "combat advantage" means and what it does and under what circumstances in particular it happens, and I would hope to the gawds not to find another "simple table"; then I'd have to look at what the standard actions are now, or more likely, what they are not; I'd have to look at the "effect forces" that moved me in the first place to see what they do or don't do, or both; and then I'd have to know what happens when I'm knocked prone and what all that entails. Granted, these aren't rocket science, but my point is that they do tend towards the "system mastery" that was the inherent problem/assumption with 3x.
Second, why not simply use the ability mods and let the player roll? If you're climbing something that requires you to hoist yourself or pull yourself up, use STR. If you're climbing a rope ladder or a twisted tree, use DEX. The DM and player should be able to come to a mutual agreement, or use the odds/evens roll to determine which skill if there's a question. If you want to use DCs, use the ability mod as the roll mod vs. the DC. Done.
Harkening back to the olden days, I recall our DM just saying, Hum, that's pretty tough--you have a 35% chance to climb over that wall, everyone would pretty much think that reasonable, or made a case for adjustment, and play continued, quick and dirty. Even yet, the thief had set percentages and was part of the class.
The latter case alludes to what Mearls was talking about earlier, about layers of complexity that can be stripped out. Climbing, etc. is the province of the class. Breaking down doors is the province of a class. Deciphering runes is the province of a class. If you want such acts available to other classes, then add the skills module.
(Finally, don't even get me started about another "invisible" layer of useless complexity, the ability scores themselves. So, we generate an ability score to determine our ability bonus? That is, we generate a number to determine another number? I would just rather have STR +4, as opposed to STR 18 = bonus of +4. For that matter, just use Fortitude, Reflex, and Will as the abilities, and have them be single digit bonuses. 4e essentially does this anyway by pairing abilities, and wizards get the same high hit bonus with their intelligence as the fighters with their strength, so basically, the to hit bonus with a magic missile vs. a fighter's to hit with a longsword is a wash--except the wizard targets a lower defense number.)
Cheers.