I'm not sure exactly what it was, but I really liked this book. I like the characters and will continue reading the series to see the other Immortals matched up (I do hope we get to continue to see Roland and Sarah). 
 I also liked that this is a slightly different take on the vampire situation - rather than a vampire being the only outcome of a bite and being a glamorized being, a mutation of the virus in gifted ones creates the Immortals who have the original power they were born with and gain others over time, while the vamps become insane rather quickly. Also, while the Immortals fight against the vampires is a key point of the story it doesn't overwhelm the plot - I didn't have to skim ahead because I was tired of it and wanted to get on with the story. 
 Ami intrigues me because she seems to be yet another type of being that even Seth doesn't know about. How Bastien changes as he's told the truth about the Immortals and vampires will be good to see - he's obviously a good person and just beleived bad information for the last 200 years, which will likely be difficult to change.