After such a long and annoying hiatus, the writers for Supernatural offer up a heart-wrenching slice of the Apocalypse. Considering the fact that I'm not much of a fan of zombie stories in general, the simplicity and focus of this story more than made up for my lack of enthusiasm.
The rising of the dead is tied directly to the coming of Death several episodes earlier, which is a good thing. Not for the characters, so much, but for the progression of the season arc. A lot of fans felt that the writers were stalling based on the possibility of a sixth season, and some are still convinced that the lack of much Apocalyptic action is a sure sign that Kripke and Gamble intend to stretch out the natural end of the story into another season.
I think it's more a matter of budget, and trying to take something as expansive as the Apocalypse and bring it down to a more personal level. The next phase of the end of the world still relies on Dean taking up the mantle as Michael's Sword, and Sam becoming the vessel for Lucifer. Until then, it's just going to percolate. And since the Winchesters have always been running around in the "sticks" as Hunters, it makes sense for matters to play out in that kind of territory.
It also makes sense that the Winchesters' allies would ultimately be targeted to apply pressure on them. Forcing Bobby to relive the worst moments of his life (or choose to allow himself to be killed) is one hell of a shot across their bow. It was perfectly designed for maximum psychological damage. Judging by the look on Bobby's face at the end, it's definitely took its toll.
Bobby has been the last remnant of family for the Brothers Winchester for quite some time, and he has been sorely wounded already. Not just in terms of his paralysis, which could have happened at any time over the course of his career as a Hunter, but also the realization of what the brothers have done. Both brothers were instrumental in bringing about the Apocalypse, after all.
In terms of story structure, Sam and Dean need to have their support structure removed before the endgame. The entire series has been a methodical process of increasing the pressure on the Brothers Winchester while killing off anyone and everyone who can help them. Bobby may have physically survived to this point, but how much more can he be expected to take?
This still makes Bobby a strong candidate to become some sort of temporary vessel for God (if not John Winchester or even Castiel). Bobby may be to the point where he's willing to make that kind of sacrifice. After all, it's hard to imagine that Sam and Dean will find a way to resolve the Apocalypse on their own. Even taking possession of all four rings from the Four Horsemen doesn't sound like it would end the problem, so much as stall it further.
Another thought came to mind during the course of this episode. The writers are going to some length to hide the appearance of Death. He is described as a tall, skin-and-bones figure by Karen in this episode, so that would seem to eliminate John Winchester as a potential candidate. Unfortunately, that was the one choice that would have been the most shocking, especially if the brothers would have been forced to kill him. The fact that Death has been off-screen thus far still suggests a familiar face, so it's just a matter of who the writers pull out of the woodwork.