I really have to reiterate that we often had to refer to walkthroughs because we got lost in what we were supposed to do. I don't think quest markers should be necessary, but I think the quest log should provide a strong sense of direction. For example, when you're stuck in the prison it might say something like, "Explore the prison to find a way to escape." and then update it with any leads you find. Combine this with a good map with landmarks labelled and as long as the map isn't too big, I think quest markers will be unnecessary.
I remember one time there was a quest where the clue was that we had to find a cave that was north of a beach... And we were like -- which beach? 70% of the map is north of a beach. >_>
