What went wrong with X-men apocalypse
1. They had no clue what they wanted Apocalypse to be as a villain, They had no clue what they want the movie to be a Disaster movie or Superhero movie had they pick either one they would have gotten a better movie
2. They didn't let Jennifer Lawrence go and she was clearly done with the franchise, In fact, they should have probably let Fassbender go as well and not because he was bad but only reason Magneto and Mystique are getting attention this point is that they are the big name actors and that change the story focus away from X-men lore to showcase those actors. Bending to please Jlau hurt this movie storywise.
3. You don't put Storm, Psylocke, and Angel in X-men movie to be the bad guys, making Horsemen recognizable X-men doesn't help if moviegoers and fans aren't invested in them. Magento arc should have been Angel arc in the movie. Angel gets hurt with the X-men and leaves , Apocalypse comes to him gives him back his wings. In the final battle Archangel breaking free from Apocalypse control is the big moment.
4. The focus of the movie should have been Jean Grey, Cyclops, Storm, Psylocke and Nightcrawler, not Xavier, Magento, Mystique, Quicksilver, Havok and Beast. Jean and Cyclops get better focus but old guard keep them from truly shining as the main focus.
5. Apocalypse looks stupid, In a world where Thanos does not look horrible, there is no reason for
Apocalypse to look that bad.
6. They failed to build a good Cyclops and Jean romance
7. Mystique being the hero of heroes in the X-men universe hurts to see
The less familiar you are with X-men mythos the better the movie is for you, I don't have problems with anyone liking X-men Apocalypse it is solid action/superhero movie and has three of better action set pieces in a X-man movie The attempt to kill apocalypse in the beginning, The Quicksilver saves the people in mansion,The final battle against apocalypse were all very entertaining. So if you don't know the lore(or don't care) combine with good action I can see people liking it. But X-man comic snob in me watches X-men First Class and X-men Apocalypse(and parts of DoFP) get very annoyed these movies don't do the X-men story justice which means they really don't do the X-men justice.