3. EVOLUTION OF SAM AND DEAN?

While Sam and Dean are the heart and soul of the show. But there’s not enough evolution with the characters. Even though there have been certain chances when writers could have played with these characters, taking them down a different path, but it was cut short. The storyline of Demon Dean was dropped prematurely. Also, the writers never re-explored the Sam with the demonic powers storyline, they just swept it under the rug once the apocalypse arc was over. Sam and Dean didn’t evolve much, but their villains regressed. Now either of the Winchester could take on multiple demons easily whereas there was a time in earlier seasons where one black eyed demon could cause all sorts of chaos. One would hope that the Winchesters would build new tools, learn different magical arts, but right up until the end, they relied somewhat on luck or the powerful allies they always have such as Jack and Castiel.
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2. FILLER EPISODES

There was a time when the show would rely on the gag or the filler episodes. While these episodes were entertaining and drifted away from the main story for a while, but they were meaningful and helped in progressing the story further. Episodes such as ‘Mystery Spot’ which revolved around Gabriel entrapping the Winchesters in a time loop where Sam would witness the demise of Dean over and over. The episode was meaningful because it helped the Sam in realizing that he would need to let Dean go. That’s the main message which Gabriel was trying to impart. Whereas episode such as ‘Scoobynatural’ didn’t add much to the story. Surely, such episodes are entertaining, but they didn’t contribute much. Also, the too bright tone and a departure from what Supernatural at its core stood for. Take for example, episodes such as ‘Changing Channels’ or ‘Yellow Fever’ were hilarious, but at the core you can see the tension building, keeping you at the edge of your seats.
1. WHERE ARE THE OTHER HEROES?

While Supernatural introduced us to various different hunters over the course of 15 seasons, but it never did true justice to them. Road House was introduced in Season 2, a hangout place for all the hunters, with the addition of Bobby, Jo & Ellen, the show became quite interesting, and the SPN Universe expanded too. But soon Roadhouse was removed from the picture. The show should have kept other hunter characters around who could act as protagonists alongside the Winchesters and prove their worth at the higher stakes.
The only reason we got numerous hunters is so that their demise could move forward the story of Sam and Dean. The demise of Jo and Ellen, two powerful female characters still feels unreal. Hunters such as Gordon were turned into monsters, eventually made an antagonist. Charlie and Eileen are the two memorable female characters who were believable to fight along the boys. But sadly, Charlie’s demise was unexpected and unnecessary, they did it just to progress the mark storyline with Dean. Just think of how many hunters have lost their lives due to their association with the Winchesters? This casts a serious question of are there not any capable human hunters other than Sam & Dean? Also, let’s not forget the Winchesters have often times created more problems than they actually solved.