1. If you are a large person, and have had bad experiences at amusement parks previously, you may be anxious about what you might not be able to ride with your kids. Stop worrying! There are very few attractions at WDW that you wouldn't be able to ride.
2. Oh, and another thread reminded me - Adults may not wear character costumes into the parks, unless it is to attend the Halloween party.
3. Keep in mind that a Disney trip is not really a "vacation." By which I mean, on vacation you might expect to have some time to lounge and sleep in. At WDW, it seems like you are always gogogo trying to see everything you want, frequently in the heat and humidity and it can be exhausting. So just remember that you need to plan for some down time so you don't burn out. We always have at least one sleep in day in the middle of the week. Which brings me to my next point...
4. Get to the parks as early as you can. You'll be able to do so much more before everyone else arrives and then have time for an afternoon break before going back out for the evening activities. However, unless you have park hoppers and can do morning EMH, then transfer to a less crowded park, always avoid the park with EMH.
5. Even if you don't book through Disney, go on their website and sign up for an account. Order the planning DVD and some free souvenir maps. You might get a surprise in the mail - a PIN number for a discount on your trip!
6. Drink, drink, drink water. Even if you're not thirsty.
7. Only take old, broken in shoes, your most supportive and comfy. Don't worry about fashion. Have at least two pairs so you can switch out. Nothing ruins a WDW trip like hobbling around on sore feet the whole time!
8. If you have a kid who is sensitive to loud noises, dark, and the unknown, prepare him or her. Show them Youtube videos of the rides before you go, so they'll know what to expect. Take earplugs and a small flash light so they can feel in control of the situation. I have a mild Aspie kid and this made a world of difference. Although he doesn't like vacationing at unfamiliar places, WDW is now like a second home to him.
9. Don't feel like you have to carry a ton of stuff into the parks. I take a small bag with just the necessities. We can get water at a CS, no need to bring it. First Aid in each park means I don't need all that stuff. Usually I just have keys, camera, phone, $/credit cards, lip balm, tiny hand sanitizer and sunscreen.
10. Be willing to consider an off-site stay. There are some really nice places nearby, and with a car to get to the parks, you will be just fine. Your family will have lots more room to spread out. You will have more dining options, and can take a break from the world when needed. A washing machine and kitchen stocked with quick breakfasts, dinners, and snacks is a big bonus! Many people will tell you it's just not a Disney vacation unless you stay in the bubble, but that's not true for all. There are some disadvantages to that approach too - so don't be so quick to drink the KoolAid, and consider your family's needs. Either way, odds are you'll have a great trip!