Keys to the Kingdom: Tips for Your WDW Vacation: Comprehensive Packing List

Rainy Day Items:

A poncho for each person (plus one extra in case one rips and one for your stroller if you are bringing one)
Flip-flops for avoiding splashes in the shocking number of puddles that quickly appear OR (my favorite) Crocs! Crocs are safe and comfy for walking all day (especially with socks) but dry quickly and easily if it begins to rain.
One beach towel (for wiping strollers, arms and legs, etc. dry)
2 Ziploc bags - a small one for a phone and a large one for camera bag.

Sun Gear:

Most Important Items:

Tickets or Magic Bands
Confirmation numbers for hotels, meals, tours etc. - I usually save ours in my phone.
Medicines: In addition to any prescription medications a member in your party needs, I always bring advil/tylenol, children's tylenol, pepto bismol, dramamine (in case of car sickness on the way there or queasiness after a big ride), midol, cough/cold medicine, and cough drops.

For the Parks:

Pins for trading - see my tips in my post "Insider Tips"
Pens that have a click top (characters in costumes like Goofy and Mickey often have a hard time getting caps off of pens)
Autograph Books (believe it or not, these really aren't a waste of money - the ones they sell at the Disney parks are my favorite)
Costumes - kids love to wear these early in the day, but be sure you bring a change of clothes in case they get sweaty or itchy and want to change out of them
Stroller - even if your kids are borderline too old to use it, you can put your rainy day pack, cooler with water bottles, etc. in it and park it and come back for it.

For the Pool:

Towels (if you're staying on Disney property they provide complimentary towels down by the pool, but if you're there in peak season there may not be many readily available)
Flotation devices for young kids
Swim diapers
Flip-flops
Bathing suits

Toiletries:

Electronics:

Free Apps:

Disney Parks Wait Times free
My Disney Experience: this one is the best - it allows you to share your reservation info with anyone in your party that has a smart phone. And it can store all of your reservation numbers. It also has wait times for each ride, including Fastpass distribution times.

Clothing:

Pajamas
Comfortable walking shoes
Flip-flops
Outfits (we pack an outfit for each day rather than just shorts, pants, and shirts - it makes matching easier, especially with young kids)
If you are going between Halloween and St. Patrick's Day, I recommend bringing a sweatshirt, even though you may not need it.
Socks
Underwear
I always have my family pack extra socks and underwear (while everything else seems to dry these never do).

For Babies:

Diapers
Wipes
Diaper Rash cream
Bottles
Formula
Sippy Cups
Pack 'n Play (I was once at a hotel where they had none available and vowed never to let that happen to me again - I always recommend you bring one if you need one).

Extras:

Soft-sided cooler for bringing water bottles, juice boxes, and snacks into the parks. (If you have a stroller, this is a great idea. If not, it's not worth it to tote a cooler around just to save some cash on snacks.) You can get free ice at your hotel. And you can get free cups of ice water from any counter service restaurant all day long.
Snacks
Drinks
If you're staying in a hotel room and not a condo and you have a light sleeper in your group a sound machine can be a life saver! Amazon.com sells some for under $20 and you can find them at your local Wal-Mart for about that price, as well.
Needle and thread never hurt to have on hand in case of an emergency.
A spare bag (if you're flying) or extra room in your suitcase for the souvenirs you will inevitably buy.

Keys to the Kingdom: Tips for Your WDW Vacation