Packing accessories like cubes, folders, sacks, etc that keep everything in its place are fantastic. I feel like these items practically doubled the amount of stuff I can fit into my standard luggage combo of a 21" rolling suitcase and a large laptop backpack. I cannot recommend these items enough. Eagle Creek makes tons of great stuff. eBags also has some less expensive options that are still made well. (eBags also has tons of color choices, excellent for families who want to keep organized! Color-code your kids!)
Most of the time on non-WDW trips, I travel on Amtrak and use a lot of public transit to get around. As a result, being able to handle all my luggage myself without assistance is fairly important to me, so being able to pack for almost any trip in that combination of bags is great.
WDW is actually a little harder because of the 'extra stuff' I don't take other places. All the hot-weather gear, the metric ton of sunscreen, the extra ponchos, the Mickey ears, the pin trading stuff...

But since WDW trips are pretty much the only time I fly, I just check the suitcase (I fly JetBlue and occasionally Southwest so it's no cost to do it) and bring a tote bag full of the 'extra stuff' as my personal item.
I usually toss a few detergent pods into a Tupperware so they don't get crushed, and a few dryer sheets in a Ziploc baggie, so I have them just in case I need to do laundry. But unless I was going to WDW for longer than 7 days, or anywhere else longer than 10 days, it probably wouldn't be necessary.