One of criticisms that seems to be quite common on this forum is that Oblivion was very flat and the fact that one could see the imperial palace from anywhere on the map made the world feel very small as a result. Bethesda has said that Skyrim will be a similar size and added that the mountain ranges will make the world feel much bigger. To get to some areas one will have to go over (or in some cases through) a mountain range. Therefore the mountains of skyrim serve a purpose in not only accentuating the rugged nature of the landscape and it's peoples but also in contributing to the scale of the world itself. I believe that the person you quoted was implying that levitation in Skyrim and would thus make the world feel as small as Oblivion and would make this effort on the part of the developer pretty much all for nothing.
Your metaphor is in no way relevant in this instance.
If a game has truly open world IMO it means that you can go anywhere you want in that game, or you can at least try. When developers are designing the open world they must bear in mind that they can not create any areas that the player isn't meant access. By this I mean that the developers can't use any artificial physical boundaries (like walls or mountains) that the player isn't meant to go over.
Now when you have a land that you are free to explore, and you know that there are some hidden areas for you to find, its nice to know that "if you see it, then there can be something." For example in Morrowind, cause of the fact that there is levitation, you can assume that you can explore all the areas you can see and there are no boundaries. But in Oblivion you get the feeling that "I'm not meant to be there and thus I gain nothing by exploring that place" when you see a steep mountain or a city wall. You go into a cave, look at the roof and wonder "could there be something to explore and to find?" In Morrowind there was something to explore and to find, but in Oblivion you think "Im not meant to go there, thus there is nothing to find."
So, its OK to use mountains as mountains, but the problem arises when the developer uses the mountains as boundaries that you aren't meant to go over in an open world game. Its like the developer is bluffing and thinking that the player won't call their bluff.
This way thinking code2501 was correct with the metaphor. If you treat mountains just as mountains, then there is no way to justify the absence of levitation with them. But if you think that mountains are not just mountains but actually boundaries that the player isn't meant to go over, then of course it leads to the game design fact that the player must be restricted in a way that he/she can not break these boundaries.
If you are making an open world game you can't limit the player too much or it won't be so open anymore.
In my opinion, levitation and the world design it leads to isn't a necessity. But this means that the absence of levitation must be compensated with other features that can not be implemented otherwise. With Oblivion it felt that levitation was taken out just to cut corners and not because there was something to gain by designing the world more 2D and restricted. With Skyrim it might be that they are really going to use the benefits. My point is that if you take levitation out, then you must make it worthwhile for the player also. Hoping for best, fearing the worst.