We have personal oxygen units that we can use if we so choose at the summit... they make a big difference in cognitive function. I try to use them whenever I am doing something especially tricky or high risk. I have made DUMB mistakes because of the altitude... (For instance I turned on a 6 kW flashlamp powered YAG and forgot I had turned off the water supply to the YAG.... The flash lamp didnt just explode- it turned to powder....)

Current weather conditions... look at the pressure graph:

Pressure up there is 60% of the pressure that you have at sea level. This leads to subtle differences in everyday activities... For instance, my technique for cleaning optics had to change when I started working on the summit. Evaporation rates of ethanol (the solvent I use for cleaning, methanol is what I prefer- ) are quite different at high altitude and low temperature.

I also drink huge amounts of water up there... It is easy to forget to drink water- but it leads to killer headaches.

Tempers flare easily at altitude...

What really astounds me is this place: http://www.eso.org/projects/alma/introduction.html

The Atacama Large Millimeter Array (ALMA):

altitude: 17,000 feet! How was it built?!