With the events that transpired at the end of book 5(Order of the Phoenix) comes a realization for both the characters in the book as well as the readers: the end is fast approaching. And with that comes a change in tone for the entire novel. Actions have larger implications, structure is breaking, and the ever-precarious divide between the wizarding and non-wizarding world is fallen. It is probably for this reason that Half-Blood Prince(along with Prisoner of Azkaban) is my favorite of the series: it's bleak. Hogwarts becomes less a place of wonder and learning and more an escape, a refuge from hostile climate and severe ramifications. And by the end of the novel, we see that even that's beginning to fail.
A major component of Half-Blood Prince that I love is that it really explores the evil of Voldemort. It is a popular device these days in telling a story from the perspective of the antagonist:Wicked, the Big Bad Wolf, Maleficent, etc; however, Rowling approaches explaining Voldemort differently. She doesn't sympathize with him or give an explanation for why he is what he is. Rather, she explains the grounds upon which a monster like the Dark Lord exists, very much akin to what Quint(Robert Shaw) did in Jaws. During the spectacular scar-showing contest scene, where Quint re-lives the sinking of the SS Indianapolis, he raises the tension of the present danger by recounting the past. Rowling does something similar with Voldemort, where Voldemort is not discussed directly, rather, we delve into his background and see the roots that developed into his evil. We learn about his ancestry and the history of pain and malice he comes from and then we are re-introduced to the present danger of the book. Learning about the enemy with Harry makes the inevitable face-off more immediate and frightening. Rowling does a fantastic job of building tension throughout the novel, concluding with the final details and a grand precursor to the final chapter, The Deathly Hallows. (I'm trying not to spoil too much.)