I actually quite like there to be different areas on the world map that host higher-level or just innately more dangerous creatures. A deathclaw, for example, is a highly dangerous creature - in Fallout lore that's one of the most dangerous monsters in the Wastes. I certainly wouldn't expect to find all monsters at every location, spread evenly all over. And some creatures are just going to be more dangerous than others. It would seem odd to me if a molerat and a deathclaw were equally difficult to kill.
So given that - isn't it logical for there to be areas that contain more difficult enemies? I could, of course, be completely wrong about this - but I can't imagine many people would argue that I should be able to step out of the Vault at level 1 with basic gear and be able to straight-out tackle a deathclaw with no preparation. These deathclaws (or any higher-tier enemy, really) need to be found somewhere, so by it's very virtue you'll have places that are more difficult for the beginning adventurer.
Me, I run into a place on the map that's too dangerous for me currently, I don't see that as an arbitrary wall - I see that as a goal. When I tried to take a shortcut through New Vegas and wound up in deathclaw territory and had to limp right back out of there I didn't see that as limiting my exploration, I saw that as I place I knew I wanted to come back to when I was properly prepared to tackle those enemies. It was with some sense of accomplishment when I was able to go back there when I was ready and take out the Legendary Deathclaw found there, clear out the area, and reap higher-tier rewards for my diligence.
If no places were more or less dangerous than any others, then I'd have been lacking that sense of accomplishment, I think. As it happened, it gave weight to my character advancement and a purpose to strive for.