Sure, this is for the most part a comical episode, but we do get a glaring look at Dean's two biggest fears: Going back to Hell and Sam going evil. It becomes obvious when he sees yellow in Sam's eyes even though the hallucinations have stopped that evil Sam is the stronger of the two. It gives us a good understanding of why Dean has been so hard on Sam. By using his powers, that fear is coming true.
It's The Great Pumpkin Sam Winchester/Wishful Thinking
These are the PTSD, Dean-starts-to-remember-Hell episodes. Now he's pushing Sam away because he believes Sam wouldn't understand. Oh boy, these guys are really brothers, aren't they? "You wouldn't understand" is a great defense mechanism for "I don't want to deal with it." Dean's drinking amps up since the flashbacks he's getting are shaking him to the core.
Alas though, Dean see's Sam's powers in action against a powerful demon like Samhain and with stunned expression must accept there's no going back for Sam. It bothers the crap out of him, but he has a better understanding of why Sam went there. He just saved the world. From this point, he backs off on Sam using his abilities.
There's also a big shift in Dean's relationship with Castiel. They gain a bit more mutual respect, especially with Castiel being honest over things to come at the end. Dean has never liked being fed bullshit, and Castiel did a ton to earn his trust.
I Know What You Did Last Summer
I bring this up for the one exchange with Sam. Dean still doesn't understand why Sam trusts Ruby so much. "I come back and you're BFFs with demon." Sam tries to explain she helped him go after Lilith, but that's not enough. "Well thanks for the thumbnail, real vivid. You want to fill in a little detail?" "Sure Dean, let's trade stories. You first, how was Hell, don't spare the details."
Dean does finally calm down and gets the honest truth out of Sam. He finally figures out an approach with his brother that works. Too bad it doesn't last.
Heaven And Hell
Setting aside grace in a bottle and Dean nailing an angel he has absolutely no chemistry with (I'm still pushing for Anna and Castiel in season five though), Dean has a ton of crap to deal with here. The angels threaten to send him back to Hell if he doesn't give up Anna. This is where Dean glowingly passes one of his big tests. He doesn't give in and calls Uriel on his bluff. He is willing to go back to Hell if that's what it takes. Of course everyone knows Dean's weakness, and the threat goes over to harming Sam. Family wins in the priority game and he gives up Anna. He is forgiven though by Anna for his choice.
While I'm still uncertain of Dean's part in the plan of the angels and demons to get together, the fact that Dean jumps in against Alastair to save Castiel is pretty awesome. The angel has grown on him. It's that mutual trust and respect thing getting stronger.
Of course since Sam shared his story, Dean finally shares his trauma from Hell with Sam. He resisted for 30 Hell years, being carved and torn apart day after day, but he ended up becoming a torturer. It's a long overdue caring and sharing moment, complete with tears, heartbreaking honesty, respectful listening and its even better on top of the Impala. It's the purest family moment this show has had. Dean has truly grown from his "no chick flick moments" rule. Dean shares again in the next episode about how much he liked the torture, but that didn't have anywhere near the impact as this scene.
Sex And Violence
Any goodwill Dean had with Sam implodes. Of course Sam starts it because he's sneaking around with Ruby behind Dean's back again. That's a slap in the face to Dean and instead of trying to find out why he reacts with anger much like he did in "Metamorphosis." Again he alienates his brother with a biting comment rather than confronting him with the truth. "First Madison, then Ruby and now Cara. What is it with you and banging monsters?" Sam reacts by throwing the phone across the room. Not a bonding moment.
Thanks to the deliciously evil and very bored siren, they are forced through the spell to come clean in the most confrontational way. Sam's words did more damage than anything. Seeing that side of Sam, the defiance, shook Dean pretty good. At the end while Sam is remorseful but unapologetic, Dean is still mad, even though he's the one that almost killed Sam. It isn't Sam's words as much as his actions that cause that anger to linger. He brushes them off as "good" though, for at this point, he doesn't know what to do with Sam.
Death Takes A Holiday
Dean's still internalizing his issues with Sam, but it's never his nature to hold back too long. As Sam's getting ready to leave the diner, Dean acts reluctant and hurt. "You sure you want me going with you? I don't want to be holding you back." Sam still won't apologize, blaming it on the siren, so Dean drops it.
Castiel is back to manipulating Dean, as it turns out their little ghost riding thing was a setup for Castiel to capture Alastair since the building was angel proofed. He even faked the call from Bobby to Sam. When asking Castiel why he didn't ask him, Dean gets all his defiance thrown right back at him. "Because whatever I ask, you seem to do the exact opposite." He had Dean there.