Movie's focus
I think the movie, despite being so long, doesn't tell us much about the important stuff. Ok, the sexual life is a big part of a nymphomaniac's life but it isn't everything. It shouldn't have focused entirely on that.
Also, I think most sexual things could have been omitted. Don't misunderstand me, I have nothing against nudity as long as it makes a point (or the naked person is attractive but this is another topic) and I don't think the nudity tell us anything new about a nymphomaniac's sex life. We already know they screw a lot of guys. In fact, I would have liked it more if the movie showed what her preferences were, what kind of weird stuff she did, what made her sexual life special, I mean I would imagine there's so much more to a nymphomaniac's sex life than screwing 10 guys a day. And I find it hard to believe that there weren't a lot of women too.