The Edge preview makes it sound like you can still go back to the open world during the linear half. It's just that story progression in linear in the second half, which makes sense since it's a tighter way to escalate events and build up to an epic finale.Prepare for reverse FFXIII. Where everything is open at first like Gran Pulse (except moreso probably) and then linear for the second half.
That's essentially what the negative shit is about. Personally it won't bother me one bit.