It is the writing and not being able to leave things 'hanging' if it happens to be ending.
Example: You know at the end of S5? The cemetery scene? I would have had Dean crawl to where Sam disappeared. (I don't exactly have objections to the plastic army guy in the ashtray, but I would have had a matchbox car in a box at Bobby's. I would have had Dean pull it out of that keepsake box just before he went to intercept Sam/Lucifer. I would have had him pull it out of his jacket pocket as Sam/Lucifer is beating the hell out of him. He would have been gripping it tight in his fist as he lay slumped against the Impala, and Lucifer was about to punch his head off/punch his head through the back seat window. I would have had Dean grab the car out of his pocket while telling Sam that he wasn't going to leave him alone. I would have had the scene play out as scripted with Sam/Lucifer being distracted by the sunlight flashing off of the matchbox car windshield. I would have had Sam then basically play the scene as written, where he tells Dean that everything is okay as he's sacrificing himself). I would have had Dean crawl to where Sammy vanished into the pit. I would have had Dean stretch out his hand to where Sam vanished to Hell. A crane shot would have pulled back from Dean, looking out in despair to his family/friends to where we see Cas' remains and Bobby lying dead with his broken neck....
Dean would have had the 'hurt face' that Jensen excels at. He would have dropped on the ground, with his eyes closed. We wouldn't know whether he died or passed out. It would have faded to black.... "End Credits right here"
In Ep 1 of S6, we would have joined Dean (no explanation at first) with Lisa. We would have gotten what *actually* happened after, like, 20 min. in, during a dream/nightmare sequence by Dean. He would have been sitting in the dining room, at the table, thinking on what happened (i.e. Cas being resurrected by *God*, and bringing him and Bobby back, but not his brother). Lisa would come into the kitchen and ask him to come to bed. He would have given her a sad smile and told her "In a minute, Babe".
She would have turned away and we would have closed in on 'hurt face' Dean. Then... Only THEN... would we see Sam standing under the street light seeing Dean sitting in the dining room/kitchen drinking milk and giving that haunted look....
I would have waited until S6 for any of this, so the fans angsted over the whole summer about Sam's, Cas', and Bobby's fate.
It worked for ST:TNG S3, when Riker ordered the 'super weapon' deployed against Picard aboard the Borg Cube.
And, if Supernatural wasn't renewed... well, the fans could have filled in the story from there. I think it was a mistake to reveal Sam's "presence" so soon right after he dived into The Pit. It was like Sera/Eric didn't trust that they'd be renewed, and so made sure that we knew that Sam hadn't been trapped in Hell... but we shouldn't have known that. It should have been an awful and horrible ending....
Yes. Like when Buffy jumped off that tower and fans knew that the WB wasn't renewing and we weren't sure if the series would be continued. Sometimes, it is right to piss off fans by not giving them a *pat* ending.
Supernatural should have ended in this manner, where we as fans had no idea if anyone survived at all.