The Rue plot in the book was very powerful, and I was disappointed that it was so highly abbreviated in the movie. I understand that a movie can't contain everything in the book and still be a civilized length, and they did a good job of trimming in general, but this one stood out as a misfire.
The book is written in the first person (first-person present tense, mostly, which is unconventional). This means that in the book you only see and know what the narrator knows. In the movie they showed some of what was going on "backstage" and I found those parts to be well-done, laying the groundwork for the political issues to come. They added rather than detracting -- not at all a safe bet when screenwriters decide to innovate.
Because of the POV, in the book the game-makers are largely invisible -- we see their work but don't see them. In the movie I thought the lead game-maker was particularly strong; seeing how what was going on in the arena affected him added a level of story not possible in the book. And oh, his final scene... nice touch.
A nit: I do wonder how Katniss was able to stay at full draw for so long, with a bow strong enough to kill a person, in that scene at the end. Especially given her state at that time. Just sayin'. (Also, what are the aerodynamic properties of silver arrows? The book referred to them as silver too, and it struck me as peculiar there too.)
The trailers I remember were: