Every season, the writers try to balance the darkness with the light, and the results are often mixed. For every smart send-up of the Supernatural formula that veils the usual excellent character study, there's a misfire.
This episode is really about setting the stage for the changes in the Brothers Winchester dynamic that will drive the rest of the season and bring closure to their overall character arcs. Sam is absolutely right: while they are stronger together, they need to act as equals to balance out their weaknesses. And while Dean (and the rest of the hunters) have been justified in their distrust of Sam after all he's done, it's good to see him gain a bit of perspective on his own role in the apocalypse.
It's easy to forget, but Dean was the one who broke the first seal, an act that allowed the whole "Free Lucifer" plan to gain momentum. As much as Sam's loyalties and perceptions had been twisted by Ruby, he still couldn't have known that killing Lilith was going to break the final seal. While this is the perfect example of good intentions paving the road to Hell, there are some mitigating circumstances involved.
Being the more draconian of the brothers, Dean was going to be the one who needed to set aside the anger and resentment the most. Dean tends to see things in black-and-white, even if he's usually a lot more forgiving when it comes to Sammy. I suspect much of it was displaced anger and self-loathing. Considering how many of his own faults have been held up in front of him lately, it's not too surprising that Dean would focus more on Sam's issues.
Sam, on the other hand, really gained nothing from his time alone, other than the knowledge that Lucifer wants to inhabit his body. Perhaps he didn't need more time to reflect on his personal shortcomings. His desire for redemption, however, has pushed him into a more self-confident stance. Hopefully it's not that he's been possessed by Lucifer already; luring Dean into giving up some control to gain advantage would be a subtle move.
As amusing as the rest of the episode tried to be, it felt like window dressing to the core emotional conflict of the episode. Unlike many other examples, the antagonist and situation didn't seem to have a direct correlation to the character conflict. One didn't feel into the other beyond the fact that Sam was right and Dean was wrong. That meant both elements had to stand up on their own, instead of strengthening one another.
Unfortunately, the whole "killer celebrity" idea didn't quite work for me. I appreciate the fact that Paris Hilton was willing to play along, but I really didn't care. Zombie Abraham Lincoln and Killer Gandhi were more fun, but even that felt forced. Add to that a number of scenes with little or no scoring, and it just didn't seem to have the pacing necessary to keep up the tension.
The bottom line is that this was the first true disappointment of the fifth season.