Sorry, but you can't use this as one of your points. The original BG games were not open world games either. I love them both myself, but if you consider either of those open world games, then you've obviously never played an open world game to know the difference.
The map was segmented for performance reasons, but if you actually paid attention you'd know that they were all precisely interconnected at every edge. You could go anywhere at any time, except for the city of Baldur's Gate itself. But even once you reached Baldur's Gate, you could still go back to anywhere you wanted.
This is what open world RPG means. The freedom to go where you want when you want.
Locking content to acts is the antithesis of this. The game is practically on rails. The term for this is arena based design. Each of the acts is a single arena that is non-contiguous with the rest.
