There was plenty of adorable Percabeth.
That whole Nico revelation blew my mind in the best possible way (I could ship it if Percabeth wasn't my OTP).
I really liked the little side thing with Calypso and Leo. It's about time the poor guy got a love interest.
Bob the Titan and Small Bob were both pretty damn cool.
Rick did a fantastic job of making Tartarus creepy and gross.
I'm not going to lie, Frank was never my favorite character, but he really started to grow on me this time around. It was nice to see him come into his own a bit.
And of course, the no HoO book would be complete with Festus being all awesome and stuff.
I could probably write a review, but 1)if you're reading a review for the fourth book in the series, you likely don't need my opnion, and 2)I'm more than a little biased towards all things Percy, so it would just be a monotonous wall of text where I did a scene by scene of everything I loved...
So, suffice it to say that I still love this series and The House of Hades made a great addition to HoO.