The fourth season of "Supernatural" was quite possibly its most consistent. While not everyone embraced the inclusion of angels on the series, I personally thought it was done well and in a manner that gave scope to the plight of the Brothers Winchester. Also, the season arc was well-paced, and the writers managed to tell a massive story within the confines of a fairly restrictive budget.
"Supernatural" earned a fifth season very early, reflective of the show's astonishing yet well-deserved growth in popularity. When the fifth season began with similarly solid numbers (at least by CW standards), the fans began to get nervous. After all, Eric Kripke had said from the very beginning that there was a basic five-season roadmap in place, and that the show was steadily progressing upon that plan.
Success, however, was bringing with it the desire for future seasons from the network. Genre fans had long since discovered that it is better to have a show end before its time than linger well past its intended lifespan. This is especially true when a series begins with intentions to tell a certain story.
It's hard to say whether or not the uncertainty of a potential sixth season factored into the decisions regarding the pacing and structure of the fifth season. One would think it impossible to ignore the discussions, especially if there was ever any consideration of extending the fifth season arc into the sixth season. Supposedly, that never was on the table, but the fifth season episodes themselves suggest otherwise.
There was also the not-insignificant matter of the budget. The five-year plan for "Supernatural" was a story of ever-widening scope, culminating in the apocalypse. That's not something easily done with a shoestring budget. While the production crew clearly did everything possible with the resources at hand, the scale of the action was going to struggle to match the inherent demands of the scope. Even within the confines of the series' premise, fighting evil on the backroads of America, there was going to be give and take.
With all that in mind, it's easy enough to understand why, after the first few episodes of the season, there was a long stretch of relatively stand-alone adventures with a comedic tone. That didn't make it any less jarring. Considering that the season was effectively one long crisis of faith for the Brothers Winchester (mostly focusing on Dean), the idea that there would be any room for comedy, much less a great deal of it, was hard to swallow.
But the main effect is the problem of pacing. While some would argue that providing the audience with a stretch of lighter-hearted moments early in the season is necessary, especially if the final half of the season is going to be very, very dark. This sounds like a good argument in theory, but in practice, it puts a lot of pressure on the second half of a season to deliver what the first half failed to bring to the table.
The net effect for this season was the feeling that the actual apocalypse arc was largely ignored for a good amount of the time, and then only brought in at the end in a rush. If one actually looks at the progression of the apocalypse arc, it is surprisingly sparse. And while plot has never been at the forefront of this character-based series, the usual balance just wasn't there.
On the other hand, it was hard not to notice that the psychological and philosophical elements of the season were well-explored. A lot of time was spent on Dean's crisis of faith on several fronts. While Sam's side of the equation often seemed to get much less development, it was strong enough to provide the necessary counterpoint to Dean's issues. Add Bobby and Castiel's arc to the mix, and it was a long and difficult look at the nature of faith.
On another level, it was also a clever way to explore the idea of "God's divine plan" in a somewhat non-threatening context. Even if one does not accept the apparent revelation that Chuck, the Prophet of God, has actually been God all along, there is still plenty of evidence that everything was going according to God's expectations.
From a religious perspective, some balked at the idea that the Brothers Winchester would be forced to suffer so much as part of a plan conceived by God, or that so many innocent people would have to die along the way. Yet, this was clearly an intentional aspect of the series. Many people question how terrible tragedies can be a part of the divine plan, but the devout insist that God has a reason for everything. If the characters question this notion, on very strong terms, it is a reflection of reality. (Besides, there will always be those who demand that all stories adhere to their world view, and cry foul when it doesn't happen.)
If the structure and overall balance of the season wasn't a reason to criticize this season, or even the religious aspects, then many pointed to the season finale. Kripke insists that this is essentially the way he had always planned to bring the series to a close, had it ended with the fifth season, so the lack of action (especially compared to previous season finales) left some disappointed or outright angry.
Of course, there are those who felt that the season finale was rightfully focused on the Brothers Winchester, as it was their journey to take. More to the point, if this was supposed to be an epic story that would eventually be collected into the "Winchester Gospels", then it makes sense that it would all hinge on their mutual emotional journeys. In essence, that is what happened in the season finale.
But there is now the question of where the series can go from here. The main story is told, and without a solid end date, development of a new multi-season story arc is nearly impossible. If they had announced that this was the final season, then at least the writers could develop a single-season arc to wrap up the loose ends and give the story a nice, long epilogue. But the possibility exists that "Supernatural" will end up in "X-Files" territory, limping along season to season until the end. Hopefully Sera Gamble and her writing staff, now that they are in control, will keep the pitfalls in mind and do everything possible to make it all feel cohesive.
The fifth season of "Supernatural" earned a Critical Myth rating of 8.1, which is still well above average. It is also a slight decrease from the rating for the fourth season (8.2). This is not much of a difference, but it does make one wonder what would have happened if the tone of the season was more balanced, and if the budget had been there to give the story the necessary scope. As it is, "Supernatural" has been one of the most excellent genre shows for five seasons running, and in many respects, it is still the most overlooked.