A book is NOT a screenplay.

Rating: 8/10 (26 August 2012)

...and that's why this movie deserves an 8/10 for being entertaining and believable throughout two and a half hours.
One of the more detailed reviews (the one by 'emptygravity') will help you out with some of the nuances that you will have missed if you haven't read the book.
There are a lot of bad reviews that claim that the movie is a "bad adaptation" - It is not. A book can be a basis for a movie, but it is NOT a screenplay and it never will be.
It's an interpretation of an artist, and his point of view is subjective and restricted - so is a photograph, that portraits somebody (literally) in a different light and so is a movie. This includes:

the cast - the plot - on what to focus - (and lots more)

But about the 'focus': A story that takes days to tell in a book can never be completely 'converted' into a movie, unless it's something as dull and shallow as 'Twilight'.
So forget about the book for once and enjoy the impression that Suzanne Collins and Gary Ross are giving us. And yes, she also wrote the screenplay, so blame her for the adaptation :)
I haven't read the book and I have a lot of unanswered questions. But that doesn't matter. The movie is just amazing - and the producers' decision to avoid the R-Rating is completely understandable in my opinion, because I see a lot of educational value. To avoid spoilers, I won't go into detail, but it's a movie that portrays a likable hero, which a lot of the movies are lacking nowadays.
You find yourself in the protagonists point of view a lot and you really start to root for the good guys. The movies portrayal of entertainment, the power of television and the media over people (and how to take it back) is only a small glimpse of what the book might have to offer.
I am definitely going to read the book now, because I want to know the rest of the story.
A must-watch on my 2012 list, definitely Top 3. Ignore the bad reviews that refer to the books. People need to get that books are not screenplays - not only for this particular film, but for all the others out there.
Enjoy an amazing piece of work.