As far as the characters go, The idea of a high level party walking happily through a low level one is something that happens even in the Big MMORPGs. And by the same token, you could have low level characters walk into a family of Beholders.
The dangers of the world, I guess.
But in most cases, I would assume each dev would have some areas for even low level characters. The biggest thing would be to allow characters to roam freely, just like they can in MMORPGs. Having a world with a linear storyline would be pointless. Obviously, when designing these worlds, heavy emphasis would probably need to be put into the NPC development and interaction, as opposed to a 'traditional' story driven game. Quite a task, but I think it can be done.
What would be great at this point is some CocoaSpud type input on this idea.