What really grinds my gears is open-ended stories.
I love Cowboy Bebop. It is in my Top 5 Anime. But I DESPISE the ending. I consider it laziness if you don't write an ending to a story and leave it open to interpretation. It's always bugged me when people will say "I'm letting my readers create their own ending".
NO!! You are the writer. It's YOUR job to write the ending. It must be canon and it is ONLY canon if you wrote it. Otherwise, its fanfiction and to most people who consume this media, it's not considered a "true" ending. It's deeply unsatisfying and I lose respect for creators who do this.
The only answer the creator of Cowboy Bebop gave was during a interview where he said, "to those who think Spike lived...they are probably right." That probably thrown in there really bugged the shit out of me because even years later he STILL hasn't committed to any canonical ending and therefore never will.
Another thing is stories with unhappy endings. I love dark stories. They can really send you through feels....but why does the ending have to be equally miserable? I can't see writing a dark story and than have the ending still be unhappy too. Want to kill nearly everyone in the cast? Sure. Go Game of Thrones on it, but please have a happy ending guys!?
The world is miserable enough as it is, why does our imaginative worlds also have to end terribly?