I think the series is definitely taking a darker turn. There are still, of course, the humor and the fun and interesting twists to traditional aspects of Greek mythology but I think on the whole, the Percy Jackson series is definitely not as light-hearted as it used to be. It's also a bit more grown up. We see Percy and Thalia have some sort of tense rivalry between them and I see it as great character development. We all love Percy but he's not the only Big Three demi-god. Thalia is a daughter of Zeus, she is as powerful as he is, and it's sort of nice to see how he works to overcome such obstacles. 
 The Titan's Curse brings with it yet another deadline for Percy to complete a mission. I liked that it introduced a whole new set of characters such as siblings Bianca and Nico, Zoe, Rachel, even the goddess Artemis. I really found it interesting that Riordan decided to portray Artemis as a 12-year-old girl. The only Artemis I've read in novels is in Sherrilyn Kenyon's Dark Hunter series and her Artemis is definitely more womanly and.. kick-ass, shall we say? Not to say that Artemis here isn't kick-ass, just considerably less so. 
 I say that The Titan's Curse ups the dark factor notably because while there's been death before in the series, I don't think it had a strong presence such as it did in this novel. I had to admit, I teared quite a bit at this one. I think also, I've really come to feel for these characters and it's always hard to see bad things happen to good people. (view spoiler)[And Zoe's death! I cried so hard; I was really moved at the part when she asked Artemis if she had done a great service. I immediately started brawling. (hide spoiler)] 
 One thing's for sure, Luke has one hell of a fighting spirit. He has a never-say-die spirit, I'd give him that. Whether or not his tenacity will help him fulfill what he sees as his mission, I guess we can only read to find out.