The Capital Wasteland is also open, unlike New Vegas which has all the invisible walls running through different sections of the map.
What are all these 'invisible walls' that you and others refer to? I never noticed any unnatural invisible walls, anywhere in NV. I've seen mountains and high, sheer ridges that are too steep to climb over, which are normal out in the real world, in the western US. Anyone who's actually ever tried to hike anyplace with those kinds of features, knows there's plenty of times you just aren't going to be able to climb over them. So you follow the valleys until you find a way around, which there always is, even in NV. I can't think of any place on the map that you can't get to from several different directions, if you look for ways around. And all of the 'impassible' ridges also have handy passes right through the middle of them in one or more places, if you just look for them.
In FO3, they used the DC landscape, of course- and anyone who's ever been there knows it's pretty freakin' tame country- there are no steep, impassible ridges or sheer cliffs anywhere in that area. So of course you could just take off in any directoin and travel till you hit the 'edge of the world'. The deserts, canyons, mountains and ridges of the west, are another story. So, I'm not seeing any of these mysterious 'invisible walls', as some misguided and geographically-challenged people are claiming to see... just naturally impassible features that you gotta walk a while to find a natural way around.
The only place I can think of in the entire game that might be construed as an 'invisible wall', is the nasty critters to the north of Goodsprings. And even a level 1 char can get by those to reach Vegas, if they are sneaky and adventurous.