Great plot, excellent character growth, healthy values, and polished writing. This is quality.
Possible concerns:
Prophecy/Faith. There is a prophecy that a band of young dragons will end a long war. And (SPOILER ALERT!) in the end a prophet says the prophecy was a fake. This brings issues of faith and hoax to the table.
Violence. Dragons eat/fight/kill/imprison each other, humans, and beasts.
Oppressive rulers. Wars, treason, tyranny, etc.
Romance. Book 4 brings new romance to the forefront; a young dragon confesses his long-time admiration and love for his friend.
Conservatives on the structure of the family may not like that later in the series some characters end up pursuing homosexual relationships. I noticed this mainly in Book 10.