What are the most important things to bring camping/backpacking that people usually forget?

Good judgement and environmentally-sound camping practices. First-Aid kit, a plan for the trip and a plan for an emergency.

many people forget to bring trash bags Pack it in, pack it out.

You want something dry and waterproofed, like dryer lint in a sealed plastic bag, that will catch a spark easily if you can't find any dry tinder. You may not need it and you can usually find something in nature to use for this...but it isn't that hard to carry. I also recommend some basic meds for you or anyone you could find in trouble. These are basic, can be lifesaving, and are also very easy to carry: Benadryl (diphenhydramine) for severe allergic reaction and real aspirin for cardiac emergency. About 10 tabs of each. Also some real sugar or honey packets for diabetic emergency. You can get these from fast food places, and when you have a good stash you can borrow for your tea! Some sort of signalling device in case you get lost. A whistle is good for short distances, and a piece of mirror for long distances. If you can catch a sunbeam from where you are, you can reflect it back for hundreds of yards into the air. It can be seen from a plane. Also a mylar blanket. Can double as shelter help anytime, but if you need it, you can wrap in it to stay warm, also a huge foil surface so it would send reflection just like a giant mirror, even if you didn't need it for heat. These are cheap, about the size of a deck of cards, and can help in an emergency. It is a one use item, no refolding it back to its packet once you get it out...so do save for a real need. Not likely to forget, but don't forget your water purification kit, some batteries, matches, toilet paper, and hand sanitizer.

Baby wipes & Sunscreen.

Emergency (foil) blanket and whistle.

Duct tape for blisters, or anything else for that matter. Dont bring a whole roll.... just about 2 feet rolled up real tight and you will not even know it is in your pack.

salt and pepper !

toilet paper, multiple forms of fire lighting(ferro rod(s), a lighter, waterproof matches and maybe a fresnel lens), compass, GPS(if your using one), I can't think of anything else right now.

The ground cloth for under the tent. Moleskin for blisters.