I thought the parts left out from the book were minor, the changes were smartly handled, and the parts added to the movie that were not in the book greatly enhanced the story.
For example:
Left out:
Madge was left out of the movie. This was a minor loss as her family's backstory isn't interesting until after the first book and even then it's not all that important.
Changes:
Instead of Madge giving Katniss the Mockingjay pin she buys it then gives it to Prim for luck and after the reaping Prim gives it back for luck. I thought this was a much more heartfelt and personal origin of the pin. I like both versions, but the movie version works better for the sister's relationship.
Additions:
Since book is all in first person from Katniss' point of view the reader gets to know all of Katniss' assumptions and theories of what's going on such as how a kiss gets her soup. With the movie we get to see all kinds of "meanwhile" moments like how President Snow interferes with the game or how Haymitch works behind the scenes to keep at least one of his tributes alive.
Overall it was a very entertaining movie and one of the best adaptions of a book to film I've enjoyed.
