Some important points contained in the book include his linking of animal behavior and human morality and comments about the suffering and its consequent blame that become keys to his later works. Also worth mentioning are his comments in 205, Of the people of Israel. Read this section. It is prophetic. Nietzsche saw the Jewish problem in Germany as critical to the coming century. That he became associated with anti-Semitism has been unfair and a travesty.
Daybreak is a great primer for Nietzsche's later, more systemic, works such as Genealogy of Morals and Beyond Good and Evil. Many of his later ideas are interrogated here, in some intances, the arguments are even better articulated.
As emphasized in the extremely well-written introduction by the editors (who do a great job in setting Daybreak in its context among other works by Nietzsche), the main subject of the book is a critique of morality -- what does it really mean to humans when we try to strip it down to its essentials and challenge the many conventions of custom. Nietzsche does not simply treat morality as an interesting subject for a pleasant intellectual dialogue, but rather makes it clear that he is in deadly earnest about how fundamentally important it is, and how our attitudes about it create ourselves and our world. You cannot read this book passively, because Nietzsche writes about difficult concepts that are very much alive today, such as this excerpt from section 149 about the common compulsion to conform to social custom, "The need for little deviant acts":
"Sometimes to act against one's better judgment when it comes to questions of custom... many toerably free-minded people regard this, not merely as unobjectionable, but as 'honest', 'humane', 'tolerant', 'not being pedantic', and whatever else those pretty words may be with which the intellectual conscience is lulled to sleep: and thus this person takes his child for Christian baptism though he is an atheist; and that person serves in the army as all the world does, however much he may execrate hatred between nations; and a third marries his wife in church because her relatives are pious and is not ashamed to repeat vows before a priest. ... The thoughtless error! ... it thereby acquires in the eyes of all who come to hear of it the sanction of rationality itself!"
There's much more of course, and one of the constantly exciting aspects of reading Nietzsche is to experience the way he interweaves discussions of art with larger philosophical concerns. His insights into literature and music are never trivial, and he provides a series of very startling perspectives. Daybreak is not the best known of Nietzsche's works, but it is essential to anyone who wants to engage seriously with his thought.