I think the choice of director came after they decided to film books, because he had a track record in the genre.
There were a lot of people originally who thought that Harry Potter was a rip-off of common fantasy themes, but the depth and the characters turned it into something different.
I've read the Olympians books, and they're a good read. One of the 'fun' parts is trying to remember what I learned about Greek mythology many years ago, and pick up on hints. An unusual feature of this series is that the author, Rick Riordan, is also a middle-school teacher, and you can download a teacher's guide on how to use the books to teach mythology. One of the classroom projects is to make your own trading cards!
The books are well-written enough that they can appeal to adults, but they're accessible as "juvenile" coming-of-age novels for the intended 12-15-year-old audience. Beyond that, and the fact that there's "magic" involved, I don't see any hint of trying to parallel the Potter books. Fantasy fiction is popular for the age group, and indeed some adults sneer at it as kid's stuff.
One issue with the movie was that they made the cast older, aiming for the 17-year-old demographic. (Of course, even more people would comment on 12-year-olds starting out at the "camp," instead of the "school" of Hogwarts.) I'm not sure how that's going to work, or how much else they might do to make it look either too much like or too much unlike HP. We were going to see the flick today, but decided to avoid crowds who don't know how to drive or park in the snow.
"Half-blood" is used in a much different sense, because many of the Greek gods had a habit of mating with mortals. One of the [intended] themes was how kids cope in one-parent families where the "dad" has left his presents and then moved on. I'd call a group of 2+1 heroes a standard arrangement, but if I had to pick equivalents in the Potter world, it would be closer to Harry+Ginny+Neville. And I wouldn't call Pierce Brosnan a casting idea for a Dumbledore equivalent.
It was JKR who chose names from greek mythology for a lot of the wizarding characters, so that was closer to a convention and not a key part of the plot. Percy in Lightning Thief is a nickname for Perseus. And I'd blame the familiar castle look on the filmmakers -- the architecture doesn't really fit with camp "houses" for the descendents of Greek gods; I'd picture something closer to the Parthenon than Hogwarts.
I don't know how it will come across in film, but the plot felt more like Narnia than Potter. But Narnia doesn't have a single central character, which is almost a must in books aimed at teenagers who are feeling out-of-place, lonely, and confused.
Some say that movie viewers are going to get really confused when Clash of the Titans comes out next month, with a plot blurb "Perseus Sets Off On A Perilous Journey Deep Into Forbidden Worlds. Battling Unholy Demons And Fearsome Beasts, He Will Only Survive If He Can Accept His Power As A God, Defy His Fate And Create His Own Destiny." Mads Mikkelsen is Draco [!] in that one. And because Hades is the closest to the Voldemort adversary for both the Olympians series and the Titans flick, of course the Titans remake has him played by Ralph Fiennes.
