Personally, I look at how modern designers try to make their games rollercoasters of sorts - thinking like "Hey, if I spent time and money doing this quest, I need to 100% make sure that EVERY PLAYER gets to experience it!" and do the opposite. That's why we have hidden quests, hidden followers, hidden dialogues, hidden ways to solve none-hidden quests, etc. Like, there was this one quest where you need to say NO to the reward and ask for a book instead, and in that book there is a ritual inside, which VERY cryptically describes a certain in-game location. If you go to that location at a certain time of day and do certain things in a very specific part of the map, something unique happens. Still, our player base manages to find even stuff like that, somehow. But there are some things and places nobody reported of finding, yet. Maybe they did, but didn't say... But who knows. You might be the first one! Speaking of multiple playthroughs, yeah, I'd name at least 3 reasons to do that. One, obviously, is the gender thing. Playing as a woman makes a lot of stuff different, if compared to playing as a guy. Also, being or not being mentally ill changes at least 15 dialogues, there are also companions you can't pick unless you play in a certain way, and as soon as we start making DLCs these companions will give you quests and stuff... So yeah. If you want to see most of the stuff in game, you'll need at least 2 playthroughs.