I concur with Mr. Ferret! Granted, we always go with a very specific itinerary in hand, but we use it as our guideline, and depart from it whenever the whim strikes.
To avoid disappointments here are some specific tips:
1 - Don't be disappionted because you left something important at home. Make a packing list well ahead of time, check it twice, and follow it as you're packing. An ounce of prevention is worth a pound of cure in this case. Yet keep in mind that unless you've forgotten prescription medication or the like, there is little that you can't buy at WDW if you've forgotten it.
2 - Don't be disappointed that your Disney experience just isn't what you thought it would be. Know what you're getting into. Consult park maps. Research the attractions and how fast their lines build. Be aware of the crowd levels for the time you are traveling, and the amount of walking and waiting you'll have to do, and prepare for it (mentally and physically) ahead of time. Understand how much everything costs at Disney World, and start saving for the trip well in advance.
3 - Keep necessary supplies with you so each day you won't be disappointed by a "touring interruption": a first aid "mini kit" with band-aids, blister pads, ointment and headache medicine; hand sanitizing wipes for snacks and meals on the go; copies of all of your ADRs and room/ticket information in case there's a Magic Band issue.
4 - Don't be disappointed by some of all of your companions wearing out or melting down. Build frequent rest breaks, naps, and "unplanned" blocs of time into your schedule.
5 - Don't be disappointed that your companions are not all "on the same page." Share all that you know about the parks, attractions, flight experience, etc. ahead of time and let everyone weigh in on -- and become invested in -- your plans.