Invisible barriers - To my knowledge, and biased opinion
, Witcher 3 handled this issue better than most open world games that I've played. The major downside it created was the fact you weren't always sure where the map ended precisely. You would find yourself viewing your map, seeing a patch of land ahead, the mouse cursor extending to these areas (its ideal to have a cursor limitation so the player does have an idea where the world ends) Geralt would cont. moving past "explorable terrain" until you finally reached the true end of the world space, then he would quip "I'm too old for this" , " I'm gonna need a boat for that" to paraphrase a few of the things that he would say aloud.Then the overworld map would pop up implying fast travel - "you've now really, really reached the end buddy".
This is my rough explanation though that is pretty much how it worked. CDPR surrounded the regions with a 2 kilometers wide 'ring' of visible, but not explorable terrain. This as you would imagine, gave the player something to look at off in the distance and gave the world a larger feeling without actually filling in the surrounding space. For those who have played Witcher 3, you already know what I'm talking about.
The Borderlands series also has a very clever border implementation with the turrets that will shoot you dead if you try to move past a certain threshold of the map (In locations where a natural world restriction wasn't present.)
The door problem can be a pain, especially when you see so many missed opportunities for adventure and exploration! (insatiable, we wants it, we needs it!) as others have said, I believe FO4 will offer many more interiors and/or at least, many more open interiors compared to previous BGS games. We already know that downtown Boston is for the most part open and that Howard and Co. said that this was one of the toughest parts to implement during development. Any non-enterable buildings are ripe for DLC, However, BGS are known for creating completely different locations outside and away from the main world map. Still, not a bad idea if you ask me.
We have D.C. Interior-esque mods and quest mods to come to our rescue if we desire more 