So this is my first post looking at 50 Shades from the literature angle, and I think the first thing that needs to be said is that 50 Shades is Twilight fan-fiction. This article on crushable.com goes into more detail. [The article is no longer up, so I’ve linked to the Way Back Machine’s version.]
I’m not going to say that a piece of writing necessarily lacks merit when it’s fan-fiction, but this is something that needs to be understood when putting that writing into context, and there are arguments (mentioned in the article above) that say that fan-fiction can never stand on its own merit alone. That said, I wonder whether any piece of good literature can truly be said to be standalone.