I am saying the "open" world is an illusion alas the world in Skyrim is a somewhat linear corridor because of the mountains.
Also, the lack of real character options outside of how to kill things makes the game more linear. I liked Fallout New Vegas (Obsidian game) because of the way my character's skills allowed more choices other then kill no name NPCs. Skyrim even has a speechcraft skill and perk tree but 99% of the time it is relegated to the usual hack and slash sword & sorcery game trope, discounts from merchants. I love how some bandits are not agro to you unless you walk right up to them, imagine if Speechcraft perk or skill would pervent them from going agro unless they catch you stealing, etc. Little additions like this would make the game feel more non-linear. Not to mention if the entire story can be altered by your PC's skills.