[Spoilers up to 4.15 and speculation]
While this Whedon mantra has been true for several seasons already on Supernatural, it's pretty interesting that right now, It's All About the Pain has become daily reality for the characters, given Dean's descent through Hell. It's moved from being the emotional arc the characters were on - to being honed down to the thing itself, metaphor made flesh. He's not just experiencing emotional pain, he's become first the receptacle of pain itself and then the wielder of it.
It's all about the pain that Dean was given for 30 years. And even more about the pain that he learned to inflict for the last 10 years. And his fears about how that pain worked to begin to destroy his humanity. His deep seated fear of himself. What he could be again if he let himself.
And this begins to explain why Dean has lost his edge, which has been so troubling lately. The fact is that he's lost his edge in so many ways. Jensen Ackles even seems to have put on some weight, so the angles in his face are blunter, less honed, the hallows less apparent, his face is a flatter plane, less beautiful. (Though I suppose they could be achieving this same effect by lighting him very differently, not to throw his angles into relief.) His entire presence feels heavier, unanimated by the sparkle and flare and self possession of early Dean. And, with some rare exceptions, he's always covered up in so many layers, you can't see his actual body shape, it's obscured behind this veil of illusion, another aspect of his (self) concealment.
But his true edge is so sharp after 10 years of torturing in Hell, he's afraid to go anywhere near it, of being in the same vicinity with it, let alone touching it and taking it back up. So he's fallen into this edgeless, whiny, passive aggressive self that's not all there, leaving a lot of the operation of day to day affairs to Sammy. And simultaneously hating the less potent self that he has allowed to return and is now present. More foreshadowing from Yellow Fever.
The interesting thing is that this aspect of Dean is what Sammy doesn't have the slightest clue about experientially, he'll be used to dealing with edgeless Dean, softer Dean, Dean with new limits Sam doesn't like - so when their eventual confrontation occurs, Dean will have 10 solid years of Alastair certified torturing and 30 solid years of outlasting Alastair's tortures in his bag of tricks. And - if trends stay true - Sammy will be fairly dismissive of Dean.
Sammy has his demon blood to make him different. And Dean, Dean has his years in Hell. With Pain as his Superpower.
That's what makes him different - and suitable to be the one to oppose againstSam/Lucifer or whoever Sam will be at the time.