A lot of times when I hear people complain about the movie, it's because details were lost in translation. Obviously the book will have many more subplots than a movie though, and I feel those people miss the point. A movie should convey the themes and feel of the book. It should bring the main plot, atmosphere, and message to the big screen and stand on its own.
That being said, OP, your friend is being an idiot about Game of Thrones. It's easily just as good as the books (personally I'm enjoying the show a bit more now that the second season is taking its own path). Lord of the Rings is a good example too, of movies that did it right. Yeah, you don't have Bombadil (yay!) and some characters are downplayed like Faramir (aww) but in the end, they were still huge movies that crammed as much as they possibly could while still staying true to the themes and feel of the books. It was like Middle Earth came to life.
Movie that fucked things up though? I Am Legend. Completely missed the point of even the title. It started things up well, loved seeing the destroyed city and such, but then the second half completely fell apart. They cut characters when there were only 2 characters to begin with! The title is all about how before, vampires were the thing of legend and horror, but afterwards he hunted them indiscriminately, and he became the thing of their legends and horrors. Blerg, just...annoying.