It should also be as light and compact as possible since space and strength are limited. Clothing should also project the image or style of choice, because to a great extent it determines how you are perceived as a traveler in foreign lands. Like it or not, clothing is a basic form of communication. Traveling lightly upon the cultural environment demands consideration of clothing and appearance, since both are seen as forms of respect.
Many religious buildings require skirts for women, pants for men, or removal of shoes.
My recommendation is to try to look at least neat while traveling. The world's peoples may think better of you, may want to talk to you more often, and may be more likely to offer hospitality. Moreover, you will make it through customs faster and be hassled by authorities less often. On the other hand one of the great things about travel backpacking is there is no mold we have to fit. If you can just be yourself you'll do great.