After such a powerhouse of writing and execution, almost anything that followed S4 of Supernatural would have to disappoint.
Except… S5 didn’t exactly do that.
Whatever else anyone can say about S5, it is undeniably ambitious, though not quite perfect. For one thing, while the season started strong and kept the “sense” of the apocalypse building, it then runs a gamut of comedy episodes starting with #6 until #9 (4 episodes in a row!). While it is important to have a rhythm and ebb/flow to the story, the audience will lose investment into the mood the storyteller is trying to set if too long is spent off it. 5.06 and 5.07 are by far the worst offenders. Episode 6 tries to have a serious tone in the last half, but the revelation of “the antichrist” never sticks because his creation does not make sense within the established world canon and logic (how does human + demon > angel???) and goes against the previous set up via dialog and implications both that Sam is supposed to be the antichrist. Episode 7 then breaks the tone of the season further by having no mention of the end times, even though it is very very relevant this season. Why does it matter to bargain your years away if Episode 4 already revealed that not too many years for EVERYONE remain? And isn’t this witch’s spell a possible solution to the question of the season? Angels may be able to revive dead bodies but can they reverse aging? That these obvious questions are brought up or addressed just heighten the disconnect from the season’s established arc. Which is why whenever people watch this season (especially for the first time) it works far better to skip those two.
Story wise there are 3 other major missteps in this season.
1) The exact consequences of Michael winning are never brought up or explained UNTIL a single line by Castiel after the climax in conclusion. The audience is supposed to be invested in neither angel winning the final battle, but we spend an episode in a possible future showing us why Lucifer winning is bad. They even make it a point in episode 16 that most people will go to Heaven so the death toll is not much of a concern. It even remains a possibility that Dean could become Michael and go defeat Lucifer while the latter is wearing Nick’s body so we don’t even have the reason of “it would be really sad to see Sam & Dean fight” as an excuse for why it’s bad. Michael even reveals that Dean won’t suffer much ill effects if they survive. Once the plot with the Colt fails, without the establishment of the stakes, it increasingly appears like Dean is less being a hero and more being stubborn and selfish.
2) Michael’s character and motivation is very weakly explored or explained, amounting to little more than “because the writer says so.” This is probably the biggest missed opportunity as a revelation from him that maybe he spared Lucifer once long ago despite his father’s orders and this has led to the current conflict would at least be payoff and an echo to the S3 character conflict between Sam and Dean.
3) How or what Sam is being tempted with by Lucifer is little more than “because the writer says so.” Compared to the finale of S2 which had a legitimately well written temptation scene between Jake and the YED such that one could understand why Jake did what he did. That Sam had any reason to join with Lucifer besides “please stop bothering me” was the single worst problem of the season. At some point it seemed more likely that he would join with Michael (to clean up the mess) and Dean would join with Lucifer (to give the middle finger to them all).
And yet despite all this, S5 was mostly a success in storytelling with payoffs for hundreds of threads and characters, proving that a solid foundation built by the writers allows the story to take bigger risks. The Colt has been established for 4 years now, when the characters think of it, the audience has thought of it and are invested with the characters for its hunt. When it doesn’t work, while part of us can feel cheated, it nonetheless feels story-logical because a major villainous force shouldn’t be taken out that easily. (and if he could be, why not sooner?) Lucifer dying by Colt would have ultimately felt like cheating because it didn’t follow any of the themes of the show. Instead the show builds into a very powerful finale that brings home all the themes and established threads of the show:
S1) Sam was twisted by a simple spirit, and tried to shoot his brother. Now with the Devil himself inside, his love for his family can overpower it.
S2) Joy and happiness could be had by the brothers (well, as far as Dean knew) but they have to give it up for the sake of others, because they’re heroes.
S3) Despite all their efforts and hard work, one of them ends up going to Hell.
S4) Dean called his brother “monster” and told Sam to get out. Now he won’t leave even if it kills him.
And that’s just some of the obvious stuff. That there’s more there, that when you look for it, you can find more and more to appreciate, is what makes the original five seasons of Supernatural, some of the best broadcast television and prime examples of storytelling.