Reinhold Messner (first guy to solo the Everest without oxygen and generally a insane alpinist) would say that the climbing part isn't what makes these high elevation mountains hard. It is the lack of oxygen which means your brain operates in a fog and the simplest of actions like tying your shoes will take forever. And that in an environment where a single wrong movement can kill you.

Messner and others keep harping on how the hardest part is mentally going forward in auch an environment. I would add: move yourself forward in such an environment and know when to cancel the thing so you won't die.