From what I've heard - you can kill everyone and still finish the main quest - there isn't going to be a very deep main story.
Allow me to contradict you there!
Yes, you can become a homicidal maniac, kill everyone, save the world nonetheless and being the only one left to attend the after party, but that doesn't mean there isn't a deep main story at all! I assure you there is, but it's up to you to participate in it.
If you talk to everyone; read everything; explore every corner, a rich and deep story will unfold. If you're more nonchalant in your approach, you may miss some of the details. If you cut the throat of every character you meet, well, information may be sketchy indeed.
It just depends on how you play the game. That's the total freedom you'll have.
