Awful writing but here's the reason I like the story-
It is a horribly written novel but it's a great story for women on many levels.
1) Woman always want to be "the one". The one who can change him, the one who turns him upside down, the one for whom he is willing to defy man and God. And sadly, some women believe that. But even for those who don't, it's a nice fantasy.
2) Women love being swept off their feet. Christian Grey goes to great lengths to court Ana. Women like being worth the effort and Mr. Grey certainly lets Ana know she is worthy of pursuit.
3) Who doesn't want to feel shiny and new? This book is full of first love and first experiences. Again, the writing is terrible, but the author easily allows the reader to relive their firsts. She also gets us inside Ana's head and we can relate to most of what she experiences. It a vicarious dream.
4) It's naughty. Even the most puritanical have a curious side. The popularity of these books validates that it's ok to explore a darker side of sex. Women don't like feeling like cheap whores just because we enjoy sex. So you have one brave soul out there who tells a friend, "Hey, I really liked this." Because of it's popularity, it's a comfort to be able to read this particular "smut" because you know that others aren't going to judge you as harshly.
(Go to a review site if you don't want spoilers. This is a DISCUSSION board.)