This may not be popular sentiment, but I don't mind the invisible walls. Sure it's jarring, but you accept that the story takes place within a set boundary. I found that most of the time when I hit one, I already knew I was close to it and/or I was being silly and
trying to trip it. It's a necessary evil that you learn to ignore.
I mean, what else are they going to do? What I
don't want to see are unclimbable mountain ridges used as an all-encompassing barrier. In certain places where it makes sense, fine, but not defining the entire Skyrim border. That to me seems more unrealistic than invisible walls.