Nearly all travelers take a one-quart, plastic toiletry bag. This bag must be removed from your main bag and then put in a bin for examination. To simplify this operation, select a backpack with big, outside pockets that can hold the standard quart-sized Ziploc bag.
The best carry on bags will also have easy-access pockets to temporarily carry your boarding pass, ID, and everything removed from your pants pockets when walking through security line. These exterior pockets must be effortless to reach whenever you're wearing your pack, zip closed so that nothing falls out, and be opaque so that no one can eye up what's inside. These compartments should be on the belt of your carry on rucksack.
Finally, if you'll be bringing your laptop with you, make certain to choose a backpack with a laptop compartment. Laptops are required to be taken from your carry on bags and placed in a separate bin to go through the x-ray machine. Make certain that your computer can be removed from and reinserted into your pack to preclude any hassles.