This is a complicated subject, and a difficult one to comment on in broad terms because of this complexity. There are so many factors that even a very basic overview would be too exhaustive to post here.
Mountain flying is not necessarily about the highest or biggest. Good techniques work in rough terrain at 3000' the same as they do at 18,000'. Aircraft performance plays a big role, but for the most part is predictable, and planning can minimize its effect. One thing I will mention which must be considered when operating over a large density altitude range is the effect Hd has on autorotation RPM and performance. She's not going to behave at all like she did in school! Care should be taken to ensure your auto-rev adjustment is optimized, and that sufficient RPM is available at the lower altitudes at light weight.
In my experience, the greatest challenge to the mountain pilot is the unpredictability of the wind due to....
1. Uneven heating and convective instability. This can result from the sun shining on dark trees, white snow, water, grass etc., lack of sun on a shadowed slope, the cooling effect of a glacier, snow patch or water, or in some cases even the exchange of latent heat from cloud formation in rising air.
2. Topographic features, and the resulting wind properties such as mechanical turbulance, direction change, vertical wind components, demarcation lines, boundry layers etc.
The pilot must familiarize himself with these factors and their effects. Then, when he has them mastered, be prepared for Mother Nature to change the rules.
A good start for an inexperienced mountain pilot would be to read up on some of the topics Eden mentioned in his first post. Then take a course. Then be careful.
Rob asked about the Contour Crawl....it is a method of mechanically determining wind information by making multiple passes in different directions, comparing IAS vs. groundspeed, and heading vs. drift. Usually used in areas where wind information is not available from other sources (leaves, grass, water, smoke etc).