With the last few weeks being centered on the Men of Letters, we FINALLY get back to the main plot of the season- closing the gates of Hell. Kevin manages to decode the tablet and tells Sam and Dean that one of them needs to complete three trials before gaining the power to close the gates. Dean, being the angsty suicidal guy he is, decides to do the trials, the first being to kill a Hellhound. This makes sense because he knows how they operate, given that he made a deal himself and was torn apart by one in season 3. To everyone's surprise, it's SAM who kills the Hellhound, completing the trial, and telling Dean that unlike him, Sam isn't planning on dying and plans to have a life after it's all over, and wants to share the post-hunting life with Dean.
This is a great episode because it actually gives Sam something to do now. Sam was originally the main character of Supernatural; Dean wasn't even introduced until 10 minutes into the pilot, and only about three episodes of season one centered around Dean. After the season premier of season 2, though, I noticed that the seasons slowly began shifting the focus off of Sam and more onto Dean. Dean was the one who killed Yellow Eyes, saved the world from Lucifer by snapping Sam back to reality, killed Eve, killed Dick. What really has Sam done in the series? Killed Lilith and jumped in a hole with Lucifer inside him. That's pretty much the major accomplishments that Sam has done. Heck, after season 5, the series could've been called "the Dean Show" because Dean seemed to do EVERYTHING while Sam stood on the sidelines watching. But in this episode, Sam now has the responsibility to ultimately beat Crowley and close the doors, leaving Dean as the helper for these trials. The show has gone back to the mechanics of season one with the focus on SAM, not Dean, though there are plenty of issues that Dean needs to resolve on his own as well. Cas is being manipulated by Naomi, and Benny has a bloodlust now that he may or may not be able to control.
I also like how the trials for closing Hell are explained mid season instead of the last for or 5 episodes like the past few seasons. It gives the boys more time to come up with a strategy for facing the Big Bad, instead of the typical "we get a Deaus ex Machina way to deal with the problem as the season ends!"