I must admit, I was surprised the first time I heard people complaining about this. True the film is not as graphic as the book, but I felt it got across the same point without being gratuitous. Also, as you already know, I felt the film was perfect and had no interest in nitpicking.
However, considering how central violence is to this story, I think it's a really valid issue to consider. I wrote up a post on the subject over on the FTN forum, and I'm now going to expand a bit here.
First of all, I want to point out that the first book does dwell more on the culture of the games rather than the horrors - these become MUCH more of a central theme in the later books. Yes, the killing is abhorrent, but equally and not as obviously abhorrent is the Capitol's attitude towards the games as entertainment. I think our 21st century brains are really quite good at filling in the blanks of the film's discreet shooting of the killings. Kids are killing each other, the concept alone is enough to strike horror in us. However the way the Capitol watches, bets on and controls the games requires more explanation to make clear - but once made clear, we realize that this, perhaps, is the truest horror of the story. Violence has always been a part of sinful humanity. But making violence into entertainment is, in my humble opinion, the worse sin, and one that western culture has really not seen since the days of the Roman arena.
Then there is another important point. If they'd gotten an R-rating, they wouldn't be able to market it towards teens and if you can't adapt a YA book for young adults then you're not going to make the movie. It really comes down to a simple financial equation and I'd rather we get a slightly watered down first film, then no film at all. Furthermore, if you make the first film too dark, it becomes much harder to market sequels. Look at all of the famous trilogies we know - Star Wars, LOTR, POTC, Spiderman - the stories all get progressively darker. If the first film were as dark as the third in these series, it's unlikely that they would have reached quite the level of popularity that they did.
But I do sincerely hope they push more of the darkness in the second and third films, and I think they will. Mockingjay is going to be divided into two films, which allows them to keep more of the darker acts in without eating up their PG-13 quota.