We've discussed this before I think. I don't like it either. NCL says the glass doors are thick and provide adequate privacy. They do have a curtain which you can sweep across and isolate that whole part of the cabin, but then it doesn't isolate you from the cabin door so you'd have to double lock that to keep others in your cabin from stumbling in at the wrong time.
NCL's logic is this layout allows more people to do "things" at the same time instead of waiting as is the norm for a traditional bathroom. I prefer the traditional layout, but I still plan to try the Epic.
We'll know more once reports get posted from the first cruise. That will be during the week of June 24th.