We take very little with us, and I'm not a pack mule, so the kids get to carry and keep up with their own stuff. With the exception of tickets and when they were toddlers / preschool age, the rule has always been that I am not carrying, keeping up with, or looking out for their stuff. They have the choice to leave it behind in the car, hotel, or at home, or to deal with it themselves. They also know that I wont go looking for anything they leave behind or lose except for maybe checking with Lost and Found on the way out. Because of this, they usually decide they dont really need whatever they wanted to bring after all.
We stick to one small cross body bag or lanyard per person. This is for phones, money, tickets, a small snack like a granola bar, an autograph pen and book, trading pins, etc. Only the items we will actually use and need that day at the park.
Then we do one big beach bag for the whole family that stays in the locker for all those just-in-case scenarios. This bag has the first aid, medication, blister Band-Aids, sun tan lotion, rain ponchos, extra clothes/shoes/bathing suits if needed, possibly a towel, zip lock bags, paper towels or wet wipes, glow in the dark toys, Disney accessories or costumes for the parks, etc. Anything we feel we might need on a one time or emergency basis or something we need for a special event or meal for that particular day. Sometimes, we skip the big bag altogether, but that usually ends up turning into the day it rains or something happens, and I wish Id brought it.
As far as the people lugging duffle bags around, Im thinking baby stuff, kids with special needs, food, clothes, shoes, blankets, jackets, folding chairs or matts to sit on, umbrellas, rain ponchos, water bottles, toys for the kids, stuffed animals, iPad, iPod, laptops, first aid, medication, Disney merchandise, Princess dresses and accessories, coloring books, and guide books. It could even be just plain luggage. Some people go straight to a park and take it with them. Others dont realize you can have packages sent back, so they carry everything they buy around.