I would point to S5 as the season where death on SPN became irrevocably cheapened. In fairness, it was already headed that way in earlier seasons, but in Lazarus Rising, Dean's resurrection still had emotional power. Dark Side of the Moon is the episode that I think made the most progress in making death a joke. Not only is the revelation of the nature of Heaven silly, but we have a Sam and Dean who are so confident in their return from death that they are actually trying to temporarily escape being resurrected. When Lucifer in Free to Be You and Me told Sam he'd just bring him back if he committed suicide, it was a bit scary. In Dark Side of the Moon it's a plot point that doesn't carry any emotional weight for me. Death is not something should be cheapened so casually.
Swan Song doesn't impress me, either. Bobby and Castiel being killed off and immediately brought back to life was also pointless and annoying. For one thing, it made Ellen and Jo's permanent deaths earlier in the season pointless and look a little sexist. It would seem the a villain's power can be demonstrated in killing off familiar male characters without their deaths lasting long than a few minutes, but women have to stay dead. It further demonstrates the increasing meaningless of death.
This naturally leads to me fantasizing about the show ending early. Problem: the first four seasons all end on cliffhangers, which makes it difficult to pick an ending. Still, S1 would be an interesting place to end it, because the show would forevermore be that cool show with lots of potential that got cancelled after the first season. Like Firefly. In my experiences, the fandom for those kinds of shows is less polarized, which would be nice.
But I would want the show to go farther than that. S2 is fantastic. The ending is a cliffhanger, of course, but it would work as an open ending. Seasons 1 and 2 are the best on an episode-by-episode basis, and both of them have interesting overarching plots. Also, S3 is pretty weak. Keeping the first two seasons and dropping everything else makes sense in many ways.
However, I love S4. Not always on an episode-by-episode basis, but the overall plot. S4 is the season that made me a Sam girl. Individual episodes are often shaky, but the overarching story is fantastic - especially Sam's arc, and Sam and Dean's relationship and conflict. It also makes S3 better in retrospect - S3 isn't great in its own right, but it works well as a bridge between S2 and S4. And if it ends on S4, there is no S5, which I approve of.
However, the cliffhanger in S4 is ENORMOUS. It's blatant set-up for the next season. One could not just drop it there.
Or could I? With just a few changes, likely mostly minor, it turns out Sam's breaking of the last seal does not lead to Lucifer walking the earth. It leads to the end of the earth. Suddenly the past four seasons are show not to be a story of the lives of a pair of demon-hunting brothers, but the story of how two demon-hunting brothers came to end the world. Everything was a build-up to this.
I would miss a great deal about the later seasons (OMG, that wink in Point of No Return), but I kinda wish this was how it had happened.