Most of the differences between Pandemonium and Delirium stem from the changes in the main character, Lena. My husband got it right when he characterized Delirium as a novel of awakening. In Pandemonium, however, she is fully awake: She knows that the government has been lying to everyone about love and emotion being a disease that needs to be eliminated, and she has come to realize that people deserve the right to feel their lives for themselves. So the Lena portrayed in Pandemonium is a much stronger, self-sufficient, and determined character than the one who was only learning to love -- and love herself -- in the first novel.
Yet at the same time, Lena still has to decide what to do with her newly awakened self. Without giving anything away, Pandemonium is about her realization that she can decide for herself who she is going to be -- that the rebellion doesn't have the right to control that, any more than the government does. So in the same way that Delirium is about her awakening, Pandemonium is something of a coming of age -- or perhaps a coming of self -- novel.
Needless to say, I really enjoyed the novel -- I was surprised but pleased by the continuing changes in Lena's character, and found the story engrossing. Waiting for the third book in the trilogy will be difficult!