Inside an act the game is not so linear. But once you finished an act you can't go back. So any quests that are still open will not be solved anymore. The starting part of the first act is the most linear one of the 4. It is intentionally done like that to guide the beginning player.
Within the other three acts the game is less linear.
It's not that bad in my opinion, it certainly reduces the chance of having failed quests.


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