[SPOILERS]
Rum Diary was rated 'R' -- "for language, brief drug use and sexuality."
The brief drug use was something of a tacked on scene (much more drinking in the book -- the fact the lead comes off as somewhat boring at times here is one problem with the film) involving some mysterious psychedelic drug provided by a crazed roommate and applied via eyedropper. Nothing much happens for a bit, then our young Hunter stand-in sees his buddy's tongue get real big and soon the two are wandering around a bit in the dark night. Johnny Depp's animated film Rango was more psychedelic by far (if to me, more boring).
The language was not noticeable. They might have used the dreaded "f" word at some point, but I did not get any sense of it being adult level language as a whole. As to "sexuality," how that even gets you a rating where you cannot go with an adult if you are under seventeen is unclear. Notice it didn't say "nudity." Nudity is hinted at times, but you never actually see it. You do see a quick scene of sex in the distance without anything being explicitly seen. Another sex scene is interrupted before it barely gets started. Amber Heard has some sexy moments here and she looks like a young actress to watch. Still, not seeing that rating here.
A bit on the story. The movie takes the basic outline of the book, removing one character and giving his girlfriend to the no principle money guy that tempts our hero,* the usual sort of movie compression as was omitting Paul's actual plane trip into Puerto Rico. A local watering hole that provides the opening scene of the book is also mostly ignored, a more questionable choice. The changing of the boyfriend results in some plot changes involving the girl (Amber Heard) and causes some plot confusion. His pal (standing in for someone else) comes along at one point when it doesn't make much sense for him to do so.
The overall idea of a young (but to him getting old fast) writer experiencing some life stuff in Puerto Rico, including with a colorful press photographer (Giovanni Ribisi adds some color too as a crazed crime/religion reporter) staid largely the same. As fleshed out more in a "making of" segment, the idea is the writer/reporter cannot find his voice and the experiences here makes him develop into his "get those bastards" persona. You do not really see it, to be honest. The movie promises more craziness than it really delivers, even the striking rape (we see the lead up) found in the book watered down here, down to much less sexiness on the dance floor. Depp seems strangely toned down here repeatedly.
And, the ending sort of comes in a rambling way (involving a cockfight to suddenly make some money for some last bit of justice that came off as unlikely, which they do not actually use and then a thing with a boat and ... a tacked on "happy ending" final crawl that really seemed ill advised) that I found badly done. The book was somewhat disjointed there too but handled things better (no cockfight, for one thing), without the happy ending business. It left things more appropriately hanging. The film had charms like I said earlier but in a disjointed fashion, including various supporting characters and some good scenes. Tellingly, I watched it in dribs and drabs, getting bored.
The extras were a "making of" segment that was pretty good and a rather drawn out home movies flavor documentary regarding the real Hunter Thompson editing the work and trying to get (a decade ago) it made into a movie. This over forty minute segment could have been shorter.
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* Aaron Eckhart is well cast here but after awhile, has not enough to do. The same might be said for the great character actor Richard Jenkins, playing the editor. The others, besides the three cited in the text, were generally okay, but nothing special. Michael Rispoli as Sala was very good though at times he was a bit too talky.