Bacpacking as a well-known way of travel allows you flexibility with your itinerary and requires lesser expenses than booking into hotels. Sure there could be a long list that you would prepare so you won't forget any valuable and wanted items. However, as you go along with your list, there are things which you might not consider bringing:
Large guidebooks - better do your research before you go and print out or photocopy information needed for travel.
Laptops and other gadgets - don't take anything valuable and expensive because theft is just everywhere and you would likely regret to lose things that can't easily be replaced.
Hair dryer - some hostels provide facilities like the use of hair dryers so there's no use of bringing this item. Besides, this is a bulky thing that would take up much space of your backpack.
Meanwhile, there are also little items that are worth bringing such as camera, personal music player, car seat back organizers, flashlight, extra batteries, 12 volt LED map light, a money belt, a small pouch or bag for use during the day.