39 Clues series. I referred to it as "a little far fetched at times" and "like a Dan Brown novel for kids." Still, I was at least lukewarm towards it, enough so that I decided to give The Lightning Thief a go as my most recent read aloud with my daughter. Sadly, the issues I had with 39 Clues were pretty much the same this time around, only I'd also charge it with ripping off J. K. Rowling. Granted, that one may be a little unfair; according to at least this person, Riordan's book was actually written before Rowling's, though hers was published first and it's not like Rowling herself didn't lift the majority of her ideas from other sources. Still, the camp in The Lightning Thief felt like a lame take on Hogwarts, while the "Capture The Flag" game was like a lame take on Quidditch (this coming from a guy who thought Quidditch itself was pretty stupid).
Furthermore, I didn't really buy Percy Jackson. There were too many times it felt like an adult trying to pretend to be a teenager. Percy, for instance, mentions Muzak on at least a couple of occasions. While no doubt plenty of teens have heard it playing, I've never met one that uses that term, or that would even know it. It's possible, I suppose, but there were a few more such references that didn't register as authentic to me.
Then there's the whole Euro/American thing. While Riordan makes some quick lip service to Christians who might balk at the idea of Greek gods and therefore risk it being banned, I suppose, in the Bible belt, he's less apologetic to anyone not from western society, pretty much ignoring them, their folklore, and their religions. Perhaps, gods from Chinese mythology make an appearance in later books, but I think I'm already done.