The whole point of having an open sandbox world is to have a lot of freedom to move around, isn't it? Generally speaking: If the game has fixed challenges, this means that large areas will be closed since they are too hard for low level characters, and later on it will be pointless to visit some places since they are too easy and unrewarding for high level characters. Scaled challenges and scaled loot means that we can play anywhere at any time, we are not forced to be in a predetermined place.
I've got to totally disagree with you here. I don't see the whole point of a sandbox game being "freedom to move around", perhaps if you had said, "freedom to attempt to move around", but in any case I think you are off in the idea that level 1 characters should be able to walk anywhere. I see the sandbox approach as a means to aid immersion, it says I can walk in any direction I choose. There are no walls, no mystical owls ushering me from one land to the next. And ES has always delivered that, but in each region of the map there should be the safer main roads and beginner quests; then more challenging places or quests that any player paying the least bit of attention will know are harder b/c of that the NPCs are telling you or b/c the landscape is covered in skeletons or all those other clues the designers put in to tell players danger is around.
So like I said, I see the open world approach mostly about immersion and player freedom. If a level 1 character decideds to blindly wander mystical dragon infested war torn nation, they yes they should have a significant chance of getting killed in a dozen different ways, but
Developers aren't stupid, even in sandbox games they limit the harder baddies be being in certain caves with lots of skeletons at the top, or past valleys with mid-level creatures in them what will make the lower level characters stop and turn back.