I have been a fan of the Percy Jackson universe for an extremely long time. I have read just about every series (Percy Jackson and the Olympians, Demigod Files, Heroes of Olympus, etc) and was really hoping for something better than the movies.
To start with characters, Grover completely throws Percy under the bus getting him expelled, and you don't really see them as friends. We then go to Percy's apartment and Gabe Ugliano is portrayed as just argumentative, and not really abusive like he is in the books. Instead of adding to how much his mother has sacrificed to keep him safe, how messed up Percy Jackson's life is, and how much Gabe deserves whats coming, it just adds a slightly dislikable character. Annabeth's casting is not what bothers me, but she should at least have gray eyes, thats literally a defining feature of the children of Athena. Grover just doesn't feel like Grover in the books. Wheres his bleating and timidity and his cap and his fake feet and his constant need to eat tin cans? The Medusa scene was absolutely slaughtered. It introduced Medusa as Medusa completely eliminating all suspense from it, they did it way better in the movie.
Next let's talk action sequences. Every. Action. Sequence. Is. Terrible. I don't understand how they could make every action sequence more anti-climatic than the last, but they manage. The choreography is minimized so when you do get an action sequence, its for 30 seconds and done. In the Minotaur scene, I didn't really feel any real risk, like Percy's mother was actually going to die (which they revealed way too early on that she didn't).
I think what gets a lot of people hyped about this show is simply that this is NOT the movies. Some people can't look past that and truly look at this series as it's own thing, the terribly written emotionless thing that it is.