When you think about it, back 20 odd years ago, the R rating was applied to anything that had moderate sex scenes, violence and coarse language. Now they've pretty much dropped everything except for the sex part, and it's reserved for the most explicit ones.

Regarding Krayz's post about 18-rated scenes and stuff, what's American Pie and GTA:IV rated in the US? I heard the latter was rated 18 there, but 15 here.