I'd say that traversing mountains should require looking for paths or more gradual slopes, it should be possible to navigate mountainous regions, but it should not be something you can do quickly, unless the game has levitation, of course. Looking at the screenshots we've seen so far, it doesn't look like mountains are things we can just walk up, but if there's a reason we'd want to climb a mountain, there should be a way to get to the top, it doesn't have to be a convenient way, of course.
As long as we don't have those ridiculous invivble walls again like we have in New Vegas, i'm happy enough with having to walk around.
I agree there. I don't mind using invisible barriers around the borders of the map if putting a visible border there wouldn't look natural, but within the borders of the map, if it looks like I should be able to go somewhere, than I expect to be able to go there. And if I end up falling off a cliff to my doom, then hey, it's my own fault for going off a cliff too high to survive the drop.