That's what I thought about the HBP line being pointless, really. If Snape had reflected the spell, walked over to Harry and just said "You dare to use my own spell against me?" or whatever he said, that would have been it, mystery solved, Snape is the Half-Blood Prince. Show, not tell, in a backhanded kind of way. Takcing on the "Yes Harry I am the Half-Blood Prince" was ridiculous and redundant. It's the kind of thing that would have some kind of DUN-DUN-DUUUUUUUUN music behind it in a spoof movie.
I generally wasn't sold on this one... I'm not much of a Potter fan regardless, but I enjoy watching movies, particularly at the cinema, and have seen all the HP films so far ( it is like a tradition for my mother and I now) but this is the first time I've ever left the cinema feeling underwhelmed after any of them. Most tellingly is that even my mother didn't really enjoy it, for many of the same reasons I didn't.
I eventually came to the conclusion that, had the entire film be a sort of 'break from the norm' of the HP films and focus entirely on the characters as they develop into young adults, relationships and all, that it would have been a better film and a much better lead-in for the next. As it was it felt like the film-makers were finishing up the script, realised they were running out of time, and shoved all the exposition in for the last twenty minutes, in a very basic synopsis-like (synoptical?) fashion. "Okay, they find a Horcrux in a cave, go back to Hogwarts, Deatheaters attack, Malfoy can't kill Dumbledore so Snape does... that covers it, right?"
I also thought - admittedly I haven't been bored enough to read the most recent book yet - that it doesn't seem to be as much of a shock when Snape kills Dumbledore... the fact he knew Harry was down there seemed to suggest he had ulterior motives in doing the deed, which I'm led to believe is because it is part of The Plan and Snape is a good guy? I don't know if that's true or not. Reading the book it was much more of a hammer blow and completely out of nowhere.