Skyrim was advertised as a non-linear open world action RPG, that is what I wanted to play. Unfortunately, in the world I feel like I am running down corridors because of the inaccessible mountains.
I'm not going to say you're wrong (even if I think you are), but I am going to say one thing;
If you could walk over the mountains... wouldn't that define linear better than, say, choosing to walk around it VIA foot path or forest?
Just sayin'

... and I lied. I am going to say it. You're wrong. Skyrim is linear as Saints Row the Third is simulation.
Smart comments aside, what are they supposed to do? Surround Skyrim with a brand spankin' new ocean that just suddenly enveloped the rest of Tamriel? Don't think that would make it less linear. They have to put the invisible walls up because... well, it's Skyrim. It's high mountains, it's winter wasteland, and it's a country on a continent.
Sorry, had to be smart again. It's a disease

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As for the rest of the gameplay seeming linear, I have but one example to run by you;
I'm running aimlessly in the forest and I happen upon a group of bandits. I get a little close, and they brandish their weapons, warning me to turn around. I turn around and leave with my life. That was a favorable experience, because usually in games, you see thieves, they see you, you fight to the death. This game, very non-linear that it is, seems to give choices in even the most unlikely places!