Yes, they said the areas are large, and that they are act-based, but that idea isn't totally new; even in BG2, some areas were accessed only as a result of story events, and not returned to.
Also, potentially, an area can be the focus of several acts at different points in the story. This would be most likely for Baldur's Gate itself.
We'll have to see, but I don't think it is negative to close off areas if the story makes them inaccessible for good reason ( like involuntarily visiting and then escaping one of the planes, for example ).
