1) Who is the half-blood prince? My vote is for Hagrid, and Dumbledore knew it all the time.
2) Who dies? Cho. Harry tries to save her but can't. Comparisons between her death and that of her boyfriend from Book 4 are unavoidable. Luna Lovegood starts to interest Harry.
3) Any other of the usual characters play a big role? Dudley pays an unwilling visit to Hogwarts to get a taste of his own medicine. The reader is actually left feeling sorry for Dudley, as he is tormented by Malfoy and his cronies. As he is about to be returned home, however, he gets revenge by kicking each of them in the crotch.
4) How many OWLs does Harry get? Not enough, but he gets some sort of special dispensation because of his heroic feats. Dumbledore bends the rules, as he has done in the past for at least one other well-known wizard.