I thought Mearls's description of the problems with Sneak Attack ("SA") not affecting certain classes of critters was an interesting take that I hadn't heard before.
We all know that "Combat Is The Only Option (and Measure)" is not a great way to design the game, so the complaint that, say, undead being immune to backstabbing makes the thief a sub-optimal choice for the game is a bit...milquetoast. Okay, if you play a game defined by combat, yeah, but that's a pretty extreme corruption of the core loop of a D&D game (Town -> Wilderness -> Dungeon -> Treasure -> Return).
But Mike's point is more subtle than that. His point is that SA not being useful against certain classes of critters means you're not thinking like a thief!
I think I can say, without too much controversy, that D&D thief is an opportunist. When you roleplay a thief, part of what you want is to think sneaky, think about the little edge you can get, think about looking for chinks in the armor, weak points to stick a dagger. In many ways, the player using their move action to flank represents that. "I get into position, and spend an action to do that."
But with blanket immunity for some critters, you're instead playing an amateur D&D Linnean, classifying critters according to their anatomies. And certain classifications stop you from thinking of them in terms of combat targets. By the rules, it doesn't matter how long you look for a joint at which to strike the golem, there's no "weak point" at all. So, the rules tell you, don't look for a weak point in combat. Stop thinking like a sneaky opportunist. Just go stab the thing 'till it stops moving. The rules don't reward you thinking like a sneaky opportunist, so they don't encourage you to roleplay a thief.
I believe Mearls is on record stating that he doesn't want the 5e thief to depend on Sneak Attack to contribute to the game. I think he's totally on the right course there! But I think what has me even more excited is the possibility that a player's psychological mindset is important to them when they are designing the game. I mean, I've been ranting about this stuff since at least 2008 (a la wrought iron fences made of tigers), so maybe my excitement is a little bit a chance to point and cheer and be happy that the designers of my favorite game are on my page. But I think it's important -- this ain't about SA being The Only Thing Thieves Do, it's about how immunities shut down parts of your imagination, why they are often kind of boring in gameplay. And yeah, I can see his point there. I'm still kind of a fan of how clear immunities can be, so there's some trade-offs, but yeah, point taken: immunties are kind of a shut-down.