This is one of those episodes that really sparks a lot of thought and discussion, in particular about guilt, and the psychology of Sam and Dean.
Some had questioned why Dean was put on trial rather than Sam—Sam himself says he is the one who should be tried. But actually the episode's treatment makes perfect sense. Osiris later explains it's not about how guilty they are, but rather about how guilty they feel. And it's perfectly in character for Dean to feel guilty—as Bobby points out when he says "Who does that sound like?" Dean has always taken the weight of the world on his shoulders, and has always been very forgiving of everyone but himself. John injected steroids into this natural disposition by burdening him with responsibilities too heavy for his young shoulders, and then not forgiving him when he fails to bear them to his father's satisfaction—as we saw in Something Wicked.
Sam, on the other hand, was not made responsible for others from a young age, and not taken to task for failing to live up to those responsibilities, and is more forgiving of himself. As he says he felt very crappy about his wrongdoings for a long time—and this is evident through season 5. I know some people feel he never gave a satisfactory apology speech, but I recently read an explanation by a male fan about how this was all done in Winchesterese Guyspeak, and how it would've been out of character for them to do it with a big emotional talk. Guys in general and Winchesters in particular speak in actions rather than words. So Sam will say in Bloody Mary "You're my brother and I would die for you" rather than "You're my brother and I love you." By the same token, at the end of Sympathy for the Devil, Sam says (in response to Dean saying you chose a demon over your own brother) "I would do anything to take it back" and "What can I do?" rather than "I feel horrible about how much I hurt you." He's expressing his deep regret and his desire to make amends in actions rather than words. And in any case, Dean absolutely does not want to hear the words. Sam tries to apologize several times in the episode, and Dean keeps shutting him down. This is very much in character for Dean, and was established from the Pilot when Sam tries to apologize for what he said about their mother on the bridge, and Dean cuts him off with "No chick flick moments." He also does it at the end of Asylum, saying "I'm not in a caring-and-sharing kind of mood." They also apologize wordlessly at the end of Tall Tales, when Sam says "Dean…" and Dean says "Yeah. Me too." So a big flowery heart-to-heart about how sorry Sam is and how hurt Dean was would have been completely out of character. But the apology did most definitely happen if you're attuned to the Winchesterese Guyspeak subtext, and the proof of this is given by Dean himself who at the end says: "I know how sorry you are, I do." Dean heard the Winchesterese Guyspeak subtext loud and clear.
Anyway, Sam feels guilty and responsible through season 5 and in the end he dooms himself to an eternity of hellish torture by no less than Satan himself to set right his wrongs. He threw himself into the Pit to save the world. There can be no greater redemption. So as Sam explains at the end of Defending Your Life, after 180 years of being Lucifer's punching bag, and emerging with a permanently scarred mind, he feels like he has paid his debt.
Dean of course did also spend 40 years in Hell, but the circumstances were different. Rather than being where he redeemed himself, it is where they broke him and coerced him into committing torture himself, and breaking the first seal. His stint down under started the apocalypse, whereas Sam's ended it. So for Dean, his tour in Hell must feel like another thing he has to feel guilty about—as evidenced by his tearful confessions in Heaven and Hell and Family Remains—rather than suffering for his sins so as to clear his slate.
I loved how Jensen played the trial. He is so brilliant at playing one thing on the surface, while letting you glimpse what is really going on underneath. In this case, while he's trying to convince Osiris that he doesn't have guilt in his heart, you can plainly see how guilty he feels. It was also great to see Sam acting as a lawyer. For six years we have known that this was his dream, and only now do we actually get to see him playing the part. It was heartbreaking that Dean would rather condemn himself to death than let Sam find out he killed Amy. And that Dean is still carrying so much guilt in his heart he could only be saved by a ram's horn.