It's very simple math man. Mountains add a Third Dimension to the landscape, especially (Or actually only-if) if they're full of content, which is the case in Skyrim as we've already seen in the footage. Oblivion's and Morrowind's Mounts acted as "Directors" of sorts, making "Laymans paths" through areas. Once you learn there's a "Jump Button" they're little more than a speed bump, creating a horrifically two-dimensional world (Though Morrowind is saved by it's very vertical dungeon design).
The best way to describe it would be the Red Mountain Region in Morrowind, the only true "Mountain" in the game. Outside of Levitation, scaling the mountain takes considerable time even though the distance from Ghostgate to Dagoth Ur is about the same as Balmora to Caldera, but thankfully there are plentiful amounts of content within that space as well.
TL:DR HxW < HxWxD