I'm surprised that some of the mythos was absent, such as the entirety of the Queen's political involvement, and most of the Senate itself. Pretty much everything the movie changed or added made the story better. It's otherwise what I would expect from Frank Miller, whose style can sometimes come off as entirely lazy.
300 is certainly a masterpiece example of turning a graphic novel into a film, capturing the essence of the book while making small improvements and additions that make it more-digestible in the moving pictures medium. I doubt I'll ever read the comics again because of this, though. That's where Watchmen did me a favor, by having a different ending, keeping me attached to revisiting the books every now and then.
It makes me even more curious as to what your friend didn't like about 'em.