The big highlight of Half-Blood Prince is the death of Albus Dumbledore.
You see, in all the previous books, there was always some reason Dumbledore wasn't around to just fix everything with a flick of his wand.
Sorcerer's Stone: Called away by forged letter while Harry, Ron, and Hermione set off to stop Voldemort stealing the Sorcerer's Stone.
Chamber of Secrets: Suspended by school governors while Harry and Ron venture into the Chamber of Secrets to stop the basilisk.
Prisoner of Azkaban: Busy talking to Minister of Magic while Harry and Hermione use the time turner to go back and rescue Buckbeak and Sirius.
Goblet of Fire: To shake things up a little, Harry is taken away from Dumbledore via a portkey that was planted by a Death Eater.
Order of the Phoenix: Dumbledore DOES sweep in and resolve pretty much all the book's conflict within just a few pages.
Something had to happen so that Dumbledore wouldn't be able to provide too much help in the final book. It wasn't going to be very dramatic if Dumbledore was there to say, "Okay, Harry, now you have to sacrifice yourself so that Voldemort will regain his mortality" in chapter one. And so, in Half-Blood Prince, Dumbledore (who was dying already because he put on a cursed ring) finds out one of his students/Harry's arch-nemesis, Draco Malfoy, has been ordered by Lord Voldemort to kill him ("him" being Dumbledore; I know there were a lot of "hims" in that sentence). To keep Draco from having to perform this task, he orders Snape to step in and kill him instead, which he does, right in front of Draco.
And it is very sad.