Alpha_Medic I guess this story is the first of it's kind to show what BDSM relationship is like. That's why people love it. But, the one who actually hate the series is people who know what BDSM supposed to be like.
These people is a professional BDSM master(I don't remember the title of their work, but it does exist) . Everything about BDSM is about consent and safety while trying your best to satisfy both the dom and the sub.
Fifty shed of gray had give an insight to many people about what is BDSM, to show that this relationship exist, but yeah, people who hate it are all the one who know What BDSM is actually like better than the author.