Walking out I thought it was a pretty soft R too, but seeing it again since there's just enough gore and gruesome things to make it R. Ridley Scott or someone involved said the big split between PG-13 and R was the c-section scene, it would have had to have been cut down massively, almost to the point where it no longer was in the film, plus a few other scenes would have had to have been trimmed down (Fiefeld, Holloway being torched). I've heard that when they were filming they made in such a way that a PG-13 would be possible, like Die Hard 4 did, but opposite of that movie they decided to go dark rather than go for the money. The film looks poised to walk away with a $350 million worldwide theater gross, which is damn outstanding for an R-rated film like this, which I guess is why Prometheus 2 is in development.