I just got back from seeing The Hunger Games and I wanted to get my initial thoughts out now. As someone who has read the book multiple times, I was able to detect the discrepancies between the book and film, and I want to discuss that as well as some other aspects.
One of the greatest differences between the book and film is that since the film is not exclusively from Katniss' point of view, there is freedom to explore other areas besides District 12 and the arena, such as the Capitol and its inner workings. I thought this was a fantastic addition because it gave much more depth than just one point of view, and it gave insight into the Capitol without having to explain much, which is great in filmmaking.
The other differences are all fairly minor and make sense from a filmmaking point of view, such as Katniss' acquisition of her mockingjay pin and the exclusion of Madge. (Including her would have required backstory unnecessary for advancing the plot.) Another difference was the appearance of the Muttations, which admittedly was slightly disappointing but also understandable. I also felt that Haymitch's character lacked the depth it had in the book, and that he was used more as comic relief than anything else.
Other critiques of the film are very minor, and include subpar (in my opinion) special effects in some areas (Katniss fire costumes, to be specific) as well as some mediocre acting on the part of Josh Hutcherson. I thought he was just okay and didn't really add as much to Peeta's character as I'd hoped for. Another technical concern was some of the camera techniques, specifically fast action sequences. I felt that, especially during the fight on the Cornucopia, the was too much rapid motion and blurriness to actually tell what was happening.
Some praise for the film would be its great, spine-tingling score, the amazing Capitol costumes, and Jennifer Lawrence, Amandla Stenberg (Rue), and Willow Shields (Primrose). What was great about them is that they were very skilled in portraying raw emotion without seeming fake, and it seemed like they really embraced the psyche of their characters. Other characters that were enjoyable were Effie, Caesar Flickerman, Gale (which surprised me, since I'm not a huge fan of Liam's other work), Seneca Crane, and President Snow. And speaking of Seneca Crane, I found his implied death to be absolutely one of the greatest scenes in the film. It was very powerful and I really appreciated how it was put together.
Overall I think the film was well-done and serves as a good companion to the book. It is nothing extraordinary, but it is a very successful adaptation and I am looking forward to Catching Fire!