There's no reason to beat around the bush, Percy Jackson & The Olympians: The Lightning Thief only exists because Harry Potter is popular. That's not a slight on the literary series, as I've never read any of Rick Riordan's young-adult novels and we're here to talk about the movie, anyway. Regardless of the film's origins, it's clear from what ended up on screen that this came into being because movie theaters are a marketplace and, after the tepid reaction to Eragon, Twentieth Century Fox had no competing product to lure in the coveted Hogwarts demographic. So they scoured the bookshelves of their local Barnes & Noble and picked up the first popular, but more importantly, un-optioned fantasy series they could find.
It must be frustrating to work under those circumstances, to have to produce a product in the shadow of the colossus that is Potter. I imagine that Fox selected Christopher Columbus, who directed the first two Potter films, and essentially told him to do it again. And there's nothing inherently criminal about that. It's silly to dislike a movie just because you know its dreams were born in accounting and not creativity; and it's even sillier to spurn movie Y solely because movie X did it better. It is reasonable, though, to dislike a movie because it's just not good.
Percy Jackson starts off okay. We're introduced to the title teen right before he learns he's the son of Poseidon and that he has two weeks to return a lightning bolt he didn't actually steal, lest Zeus start a Godly war (somehow without using the most powerful weapon in the world that was just stolen from him). But this promising riff on what a hidden world with orphaned Demigods might be like soon devolves into a by-the-numbers fantasy 'adventure' that literally has to follow a map to its inevitable and obvious conclusion.