At nearly 2 and a half hours, the movie is already bulky. It stays fairly true to the original plot, straying only at some very small places. With so much story to tell, there's not a whole lot of character development, and this story is all about the characters. If you don't care about the characters, how can you care about what happens to them? How can you understand why some things happen the way they do?
As for the book, I kept waiting for things to happen that didn't. I waited for Katniss to become more than she was, and I suppose she did ultimately change (which you would never know, really, if you read the next 2 books). But what the movie revealed to me about the book was that it wasn't what happened that endeared me to it, but the emotional depth. It was the pain, despair, and horrible circumstances under which they had to act that made the book compelling. It was Katniss' confusion and her eventual defiance. It was Peeta's unfaltering goodness, his obvious kindness and strength of character. It was Katniss' unexpected bonding with Rue that made it all the more emotionally crushing when Rue finally met her end. All of these elements were missing from the movie.
Admittedly, the limitations of film almost always make an adaptation inferior to its written counterpart. How can you convey thought, and even if you did, how do you write it in without it coming out a little bit silly (like Dune, for example)? But I felt that The Hunger Games as a movie focused so much on the action and staying true to the original text that it lost the soul of the thing. And I'd heard the haunting soundtrack for the movie before I'd seen it, and I thought it successfully took me to that place of despair and darkness, and I expected the movie to take me there as well.
There were a few small things I didn't like about the movie, but I won't go into them here. It's not to say that I didn't like the movie or that it was bad. As book adaptations go, this one wasn't terrible. Nowhere near the horribleness of Eragon! Maybe if I saw it again, trying hard to see it as just a movie and not as a book, I might find it more enjoyable. Anyway, let me know what you thought of either or both. It's very likely you saw something I did not.