I think the complexity of various modes of moving in the world also contributes to the degree of openness. Fallouts 1, 2 and NV are quite open because you can theoretically just decide to go to a place and go there (you will probably die). But how freely you can actually move through the map?
Morrowind has numerous independent modes of moving through the world map. You have silt striders (the typical fast travel between cities that most games have), guild guides (only between mage guilds in cities that have mage guilds), ships, levitation, mark/recall, divine intervention, almsivi intervention etc. All of these have their own different rules, and you have to combine them to move through the world efficiently, but together they give you an absolute degree of freedom that makes Morrowind an absolute open world, compared to other open world games. What other game allows you to mark any point on the map that you desire, and then teleport to it instantly from any other point on the map? You can even enchant an item with +1000 fortify acrobatics and jump from one side of the map to another. Nothing even comes close to the absolute openworldness of Morrowind.