Mountain weather can change quickly. A sunny, hot day in summer can turn cool, windy and wet quickly. It is common to receive snow at high elevations in late Fall through Spring. It is crucial to be prepared, so you'll be comfortable and safe. Wool is great, but it can be bulky and heavy.
Fleece is superior to wool.
Again, it keeps you warm when wet and dries quickly. Please do not bring cotton sweatshirts, jeans, etc. Cotton will not dry, tends to make you cold and is heavy to carry when it soaks up water. Keep in mind that small and light are optimal. Less is more in the mountains, in that we tend to enjoy ourselves more when we carry less and simplify our needs.