I feel as though Supernatural is one of those shows that does best with the stand-alone, quirky episodes. This cannot sustain an entire season, of course, but this is where the show shines. Take, for example, episode 808, “Hunteri Heroici,” in which there is a “bubble of weird” that causes all physics in that bubble to behave like a Bugs Bunny cartoon. That said, I liked the idea that the doors to hell could be closed. It gave the characters a good arc to follow, something a little different from the typical “big bad.” Two of the three trials were a little weak, but the “cure a demon” trial was pretty awesome. Unfortunately, this was all rather predictable. The moment Sam started to feel woozy after completing the first trial, it was clear that the trials would kill him. The moment that Metatron and Castiel got chummy, it was clear he was evil.
In general, this season felt very unfocused. In addition to the devil tablet and the trials to close the doors to hell, you had the same nonsense for heaven. Then there was Dean and Benny, and Sam and Amelia. Castiel always has some drama going on, and the Winchesters had to keep an eye on him as well as Kevin. There was the League of Gentlemen, which was cool, but the Abbadon storyline kind of went nowhere. The season finale tied up all the loose ends pretty well, but it was a tough journey to get there.
Where will season nine go? The Winchesters did not finish their trials but Metatron did. Sam may not have died, but there is no chance that he is unchanged. Maybe it will be something good - give him superpowers or something. I also think that Crowley will be forced to join forces with the Winchesters to get the fallen angels back into heaven so that he can keep his station as the king of hell. And Castiel is human now...? So I guess he really will have to learn proper hunting.