The largest issue I had with the film was how much it left out. I went into this movie expecting to see much more about the past than we did. Where were young Severus and Lilly? I so wanted to see that, to get more depth from Snape's character. The book was all about him. The movie? Not so much. It wasn't Harry Potter and the Half-Blood Prince. It was Harry Potter and the Awkward Teenagers. The only mention of the Half-Blood Prince came at the beginning of the movie and the revelation at the end. A revelation that, frankly, fell flat.
It just didn't live up to it's potential or it's hype.