I never thought I'd find myself nodding vigorously while reading Entertainment Weekly, but this article hits it on the head for me:http://www.ew.com/microsites/longform/fiftyshades/
Edit: sorry, I know I should post a short passage to explain the link - it's very long, and it wanders, but in sum we've not ever in human history invited women to explain their sexuality, or explore it in any depth.
I've already read that EL James described it as her "mid-life crisis writ large", and to be honest that's the best bit for me: that this comes from her. Aside from all the other questions, it's inconceivable that (originally) she set out cynically to write a blockbuster: she just seems to have got lucky writing about what floated her boat.
I think it supports my "meta" theory about it - that people go in to perve over the actual bad behaviour of the guy, but it's ok because there's the play bad behaviour in there too.