Material that is UV resistant and smooth like a dry bag so dirt and grime doesn't cling to the surface. Good quality zippers. I like a couple external pockets big enough to hold a couple rolls of T.P. that have been vacuum bagged flat and some wet wipes. Machine washable.
Some features I'd like to see is.
Either MOLLE webbing for the attachment of an E-Tool or a pocket large enough to fit one.
A good lip/flap inside the bag that one can attache a garbage bag to.
Center strap to prevent sagging.