There isn't much left out here, i thought the character development was nice, and i really loved the interaction between Sousuke and Chidori. If you are looking for something with good animation quality and good audio you choose the right series.
What makes it so good is the characters, and it's not the standard chosen one mecha, which is a huge plus.
Another great thing about the series is how bad ass the main villain is, he is really a nut-job, and he has quite the back story with Kashim (Sousuke), and he portrait the madness perfectly, one of the greatest bad guys of all time!
[Positive]
Huge value due to second season and probably more seasons, Good animation quality, good audio, good character development.
[Negative]
They leave out a few things from the books, but not that big of a deal.moreless