I meant to do this post ages ago, but I might as well do it now.
Most of the differences between the movie of the "Lightning Thief" and the book were okay by me. I would have preferred the book version, but going to the Parthenon in Nashville was a pretty good idea for the movie.
There is one pretty big thematic element that was changed, though, that stuck in my mind long after I saw the movie. Here it is:
In the book, the gods don't have much to do with their kids. This is one of their big rules. But it isn't done out of malice or out of any true neglect. In a sense it is just the opposite. By separating themselves from their kids, they are forcing their kids--born to be heroes and leaders--to find their own way in the world. In a sense, it is a choice between the kids having a silver spoon in their mouth from the day they are born, and thus never having to fight for anything in their lives, and between the kids having to fight for every gain--making each success more worth while. It's a very ancient-Greece sort of idea, and was in line with the old myths.
The movie totally turns that on its head. In the movie, the gods desperately want to be a part of their kids' lives and to help them, but they are barred from doing so because of an edict from Zeus. Instead of a well-thought-out and true-to-the mythology concept of letting their children find their own strength, we get a post-modern concept of parenthood as coddling and protective. Only a stupid law keeps the gods away, very much against their will.
Anyway, that is the one big difference that stuck with me all this time.
(And I still want to know why the Kindle version of the last book is slightly different than the hard-cover.)