The purpose of a sleeping bag is to preserve your body heat, thus keeping you warm. Since you have to carry the bag on your bike or on your back, you'll want it to be lightweight and very compact. It should also "breathe" (allow perspiration to pass through during the night). A waterproof sleeping bag would soak you in your own perspiration! A bag should be able to dry out fairly quickly if it gets wet, and regain its loft (thickness) immediately and continuously after being crushed all day in a stuff sack.
Weight and Size
A good three season (spring, summer, fall) bag should keep you warm (with foam pad underneath and inside a tent) to at least 40 degrees. Any sleeping bag being used for trips should have a total weight of no more than 4.75 lbs., and stuff into a sack about the size of one volleyball but no larger than 2 volleyballs.