One of the true bits in the National Geographic program about the Appalachian Trail, currently streaming on Netflix, was a couple days up the trial from the South End, where most people start the hike, in Georgia, is a camping shop that also offers a package service. They got forced into this because most of the hikers filled that packs with extra crap. Neurotics chant that "two is one, one is none" and double the weight on their pack, making them extra tired, extra cranky, and quit before they see anything awesome (my dad hates the word "Awesome" because my generation started saying it in the early 1980's). This shop packs up their extra gear and sends it home by UPS. A good and needed service.
Real backpacking is about MINIMALISM, something I've been passionate about for decades. It was a primary disagreement and incompatibility I had with my soon-to-be ex-wife. A divorce is not instantaneous. I hope she's enjoying the Pacific atoll, that she got that job. She wanted to go as soon as she first heard about the place. Anyway, minimalism is key in backpacking. Carry as little as you can get away with, and you'll walk farther in greater comfort and the hike will be more what you were hoping for and less of a deathmarch, which unfortunately with my long legs, most of my friends accuse my hikes with them of being. How sad is that? Just walk faster, with purpose. There's a great view ahead, but the light will be wrong if I wait for you. Faster!
If you carry less crap, faster is possible. This leaves room for things which weren't available before. I also, in walking those long trails, recommend staying in a hotel or motel every few days, to get clean, to heal those bruises, to wash your laundry properly and get a good meal. There's no need to starve on trail food every night. A lot of backpackers show serious signs of "re-eating" starvation, where their body starts eating their muscles once it runs out of fat to keep them going. That's very unhealthy. At some point, it attacks their joints, making movement damaging too. The really determined keep going. The smarter ones rest, eat, and recover.
I wish it wasn't necessary, but a backpacker needs a gun. Considering that black bears and feral hogs can be aggressive. Bears tend to burst out of the brush without warning, and the bluff charge is largely a myth. They mostly just charge. You don't hear about bears charging and NOT stopping when the bear eats that witness, so it throws off the happy ending stories. And that's 4 pounds for a 45 or 10mm pistol or 44 Magnum revolver.
An emergency rescue radio beacon, for after shooting the bear or breaking your leg in a fall. $3000 is cheap to save your life. I plan to get one of these. This is about a pound and will call a rescue helicopter if you're too far off the roads for a truck to reach you.
Water filter is a massive yes. A light aluminum pot to boil water in too.
Emergency food? Very important. Whisperlite stoves are an awesome invention, and can be setup, used, and put away in about 15 minutes.
Remember the tiny bottle of dishwashing detergent. Grease spoils really nasty and causes some dangerously dehydrating food poisoning.
Rain poncho? Yes. Tiny tent maybe too. Depends on the hospitality options along the trail. A tent is at least 8 pounds. If you can get away with NOT carrying one, that's a lot of weight saved. If you walk the extra miles to reach the next town, you aren't sleeping in the rain.
If you plan to sleep off the trail, a sleeping pad and sleeping bag are heavy and necessary. If you can schedule your stops to coincide with motels every night? Do so. You'll be happier and healthier and while you might miss the camaraderie of camping with mosquitoes and pit toilets on the trail campsites, you'll be getting better nutrition and not be risking meningitis (West Nile Virus, Bird Flu etc).
Spare socks, and liners.
Toilet paper, in a big ziplock bag. So key. Few pit toilets are stocked or cleaned anymore. Thank Dept of Interior for that insult.
First aid kit. You might use it to help a fellow hiker. Include meds for diarhea and infection. It doesn't take much to get parasites. This is why you should visit a bar every week and get drunk. The alcohol purges most intestinal parasites. It might save your life. Learn to like quinine water and gin and tonic. Malaria was really common in the USA a hundred years ago.
Sewing kit to fix any gear damage. Backpacks can get torn. Straps can require reinforcement or adjustment or padding added to a hip belt, where most of the weight in a pack is SUPPOSED to rest.
I am also a fan of quiet packs. If you put black electrical tape on each of the pack ring pins, it stops tinkling when you walk. You may think the tinkling noise is saving you from bears, but survivors of bears running away don't tell the story of the bear going to ambush the noise because they heard it first. Alaskans carry guns for bears for very good reason. Bears have personality, and moods. Sometimes they just want to eat you.
Dress in layers so you can take stuff off or put things on. Fleece is wonderful. Serious hiking is done in shorts, but you can pull a pair of fleece sweats over it. Shirts with shoulder buttons? Bad. Think about where the straps go. The hip belt can give you awesome bruises from belt loops. Think hard about what's there. Fitting shorts don't need belt loops. Remove them and save yourself some pain.
When your clothes wear out or stink too much or are badly stained and won't launder clean, buy new clothes in towns. Nothing says you have to wear the same stuff the whole way. Use fabric softeners so the clothes won't itch or give you rashes. The people you are sharing the trail with will thank you not to stink like a dirty hippie. It's not NICE to stink. Buy soap, wash, thoroughly, throw it away. You don't have to carry it, just use it when you can.
Lights. Modern LED headlamps are cheap, waterproof, and provide adequate light for 60 hours on a set of small batteries. One of the best new things, because you can hike past dark, if you really must, and you can wash dishes, which is a massive pain using a lantern because the light never reaches into a basin. Headlamps get around this. Buy new batteries in town. Cheaper and lighter than carrying lots of spares.
Watch out for heat rash on your back. Most backpacks fit poorly and your back gets really hot, the sweat trapped between you and all your stuff. I have NEVER found a good reliable dry pack. Lots of BS claims, all of them false. The only cure is less stuff and taking it off every half hour or hour.
Superheavy packs make for exhaustion and unwillingness to unhook because that means hooking it back up and then lifting it up again. That's a special torture. Fancy frameless ones have an internal frame, which can get bent by excess weight. They also sit extra close to the skin and make you even hotter. The less gear you have, the smaller and more comfortable the pack. My preference, after years of research and testing is a basic kids knapsack, like you'd use for college classes. Just make sure all your gear fits in there, and arrange it so nothing is jabbing you in the kidney.
A clever hiker has a friend to call that will drop off gear for you along the trailheads via rental locker companies (lots of different commercial ones) so you can get the appropriate gear or new boots or whatever. Its not as traditional as just suffering, but do you want to have lifelong crippling injuries because you were too obstinate?
It is important to note that the above gear will also fit into a modest pair of saddlebags on the back of a motorcycle or touring bag for your bicycle rack. The less gear you carry, the better. Ultralight motorcycling in the 125-250cc class is a subset of motorcycle adventure touring. Very light bikes can go amazing places. Like mountain bike amazing, just much further in a day because the operator isn't pedaling. This leaves him with more energy to watch the road, shift weight, and steer before exhaustion sets in and he has to stop. I still recommend camping at a motel with hot showers and soft beds, but that's me.