Size - 6, 8, 10, 15, 20 and 30 litres. 20 litres or less is recommended for safety and comfort, especially if you are commuting through traffic. For a longer day ride a larger pack may be suitable if the contents of the bag will be gradually reduced, i.e food eaten, fluids consumed.
Fit- Padding, vents, straps, fasteners, pockets, compartments, stiffness and flexibility will all affect how the pack fits. Consider your riding style, use and body shape when choosing the pack.
Structure - Most people will have more than one use for their bike and bag. Some packs have a firm padding up the back with an internal board to increase stiffness. Some are flexible with breathable properties offering more comfort than support. Pockets and compartments can be useful for ease of kit access but they also affect how the pack fits.
Material - The type of fabric depends on the pack use. Some are more hard wearing but can be heavier and others offer more water resistance. Sometime a mix of materials is used to assist with durability, water resistance and style.
Visibility - Cycling specific packs should have reflective piping, or at least printed reflective detail such as brand logo or strips, on the main compartment, pockets or straps to help with front and side visibility. This is important when riding in traffic and poor visibility.
Additional features - Laptop compartment, hydration bladders and/or internal frames are sometimes added to enhance the packs use.