I'll be seeing this on Friday, so I'll be the judge wether or not this sucks or not. I own all 5 books SIGNED by the author himself (I've met him twice), and here's what it looks like they took out from the book in order to keep this movie self-contained from what I could tell from spoilers I've read *SPOILERS FROM THE BOOKS AHEAD*:
* No mention or appearance of either Ares (who plays a MAJOR role in the first book) or his demigod daughter Clarisse, who plays a major role throughout the series (she's sorta like Draco Malfoy from Harry Potter, but she's not as mean or evil)
* I don't think Dionysus plays as big of a role as he does in the books (as far as I can tell), ESPECIALLY in Book 3 where he saves Percy's life.
* Kronos the Titan is the REAL villain in the books and is seeking to return to full power by gaining followers and tries and fails to seduce Percy into joining him. THIS is the real reason why Luke steals Zeus' lightning bolt in the book. In the movie, it looks like they COMPLETELY got rid of all the Kronos stuff and just gave Luke a different motive in order to keep the movie self-contained.
* Thalia, Zeus' demigod daughter from the books who was turned into a tree in order to prevent her from dying, doesn't seem to be mentioned AT ALL in this, leaving out major character development for both Anabeth and Percy overall.
If anyone who's both read the books and seen the movie can correct me or add more to this list of missing/changed stuff, I'd be very grateful.