The main issue is the fact that the atmospheric pressure is significantly lower at high altitude. This causes water to evaporate more quickly, and water actually boils at a lower temperature. If that's a difficult thing to get your mind around, then you can imagine what a strange experience high-altitude cooking can be.
There are other issues, too, and they tend to increase in severity as you get higher above sea level. Read all about cooking at high altitude. Even if you don't live at a high elevation, I think you'll find it interesting from a purely culinary standpoint.