To quote a friend, "Its so cool there aren't adequate words to describe it."
But Nico... agh the angst loneliness sadness ake,rhgnjae,f I usually don't really like stuff like that, just personal preference sorry, but Nico made me so so so sad it was like watching Because of Winn-Dixie.
Also, no one died, which was really kind of disappointing. I know there's that unspoken taboo that all writers have, that you can't kill off your main/major characters (though Brandon Sanderson did it perfectly in Mistborn), but the whole book was hinting "Leo's gonna die... Hazel's gonna die... Someone's gonna die..." but they didn't die. Though, I'm okay with it because Riordan wouldn't stoop to using a "deux ex machina" and wound the plot together so that the ending was okay.
The dedication page: "To my wonderful readers: Sorry about that last cliff-hanger. Well, no not really. HAHAHAHA. But seriously, I love you guys."