"Growing up in a place like this would freak me out."
"The manicured lawns, 'How was your day, honey?' I'd blow my brains out."
"There's nothing wrong with normal."
"I'd take our family over normal any day."
- Dean and Sam, "Bugs"
Others have already pointed out a lot of the really fascinating mis en scene stuff in this episode, like the Blue Velvet homages and the recreation of the first adult meeting between the brothers from the pilot. Which were both amazingly cool, and this show continues to confound even my predisposed-to-fondness expectations.
Back when In My Time Of Dying aired, I was one of the people who genuinely thought that there was no way Dean was going to opt to go with Tessa. He's too stubborn, for one thing, and incapable of giving up if he has the option not to. There's also a self-denying streak in Dean, a "you can't have nice things", which leads him to never forming stable romantic relationships and making a jerk of himself in public on frequent occasions; he's not a guy who's going to opt for his own eternal rest over the slimmest chance that he can do some small good for someone else.
In a lot of ways, I felt like this episode paid off Dean's journey through season 2. I've been a little frustrated with the handling of Dean in this season, because I felt like he was stuck in limbo and not really being written with the kind of cohesion and intricacy that the character had in season 1, but this one tied everything back together into one arc. In Children Shouldn't Play With Dead Things, he was uncomfortable with visiting Mary's grave, but here he goes to John's and cries.
He's almost pleading in that scene; he needs absolution for the sin of wanting to be content. And then he cuts himself off, because he knows there's no point in even saying that life's not fair, that Dean's life's not fair. That's just how it is.
We see him finally making the choice that the Demon stopped him making with Tessa. Dean's been offered a couple of easy outs this season -- at the crossroads, he's offered his heart's desire in exchange for his own demise; Tessa offered oblivion; the Djinn offered a fake happy ending -- but the only one he opted to take was when Sam seemed to be infected in Croatoan. If Sam goes, Dean goes, but otherwise no death's so enticing as to take Dean away from the world.
So it turns out Dean has been on a journey in season 2, but it's been one we've only seen snatches of. He's let go of his need for a father/mentor/teacher figure; he knows he has to supply his own answers to the tough questions. It never surprised me that Dean was an atheist, because he's not someone who trusts in a benign universe, but I think that now Dean's become the kind of man who doesn't just not believe in a god, but who doesn't need a god. He'll muddle through on his own, because that's the only choice he's got in the matter.
I've seen the Djinn-reality referred to as the 'Wishverse' by a couple of ljers, which made me pause for a second, because I'm so used to seeing that term used about the Buffy episode 'The Wish'. And then I was thinking about it, and really, Dean's real life is Wishverse-Buffy's life. He's the unsocialised, tough demon-hunter with more pigheadedness than optimism, and the world is a place that's getting darker and darker, and all he can do is fight until he dies fighting. What was the grim horror-alternative in Buffy's world is all Dean's got to start with.
The Buffy episode that's most obviously parallel to this one, though, is Normal Again, in which Buffy is infected by a demon's poison which creates a world in which she's mentally ill and completely un-supernatural. In both Dean and Buffy's imaginary worlds, the two biggest things they're offered are their mothers, and a chance to rest. That's all they want. A chance to sleep, to not be the one who has to lead the charge and give up their own lives -- not just by eventually dying, but by never having a chance at normal in the first place -- for a greater good. But they each give their illusions up and return to the fight. Because that's what heroes do.