We're kind of thrust into this unfamiliar territory — well, unfamiliar for most of us — with the whole "dominant" and "submissive" thing. They talk in abstract about "rules" and "punishment" and "safe words," but we only get a few glimpses of it in practice. Anastasia never even uses the safe words.
What's more, how did Christian go from being a submissive for six years to being a dominant? Is that how that works? Do you just suddenly decide you want to switch roles? It seemed weird.