One example that might provide an idea of what will cause an R rating is the color of Klingons blood in Star Trek VI. Had those scenes featured red blood instead of pink floating around the ship, in that quantity, it would have received an R rating.
Another example is the visceral intensity around how actions are depicted. This isn't as easy to explain, but Wolverine and Freddie Kruger both have claws. How they are depicted as being used, how the victim is depicted as responding to them, and how horrifying it is to the viewer to watch, is why one of those characters typically gets an R rating and the other one doesn't (with an exception of course.)