These are your main non-waterproof trousers which you'll wear when operating in the mountains; when raining you add your waterproofs over the top, and if very cold and windy, waterproofs and perhaps even thermal leggings as well. It is a common misconception that waterproofs are always needed when operating on snow; given reasonable weather, most of the time you will just have these trousers on your legs with your gaiters and boots, so look for a reasonbly robust fabric, synthetic being much better than cotton as it doesn't absorb water, is tougher, and also more weather resistant. Jeans are definitely unsuitable.
Zip-off lower legs are OK but not ideal as if the zip breaks we have to gaffer tape your leg back on; this type of trousers also tend to be a bit too thin, so better to bring a separate pair of shorts for hot hut approach walks (see "Change of Clothing"). "Schoeller" type fabric is ideal if you can find it.