Accessibility
Disney is one of the most accessible places on earth, and people dealing with special challenges will nevertheless be able to experience most attractions in one form or another. There are a couple of areas, however, where some advance planning to manage accessibility concerns is wise:
Disability Access Service
If you have a disability that makes waiting in a traditional queue problematic, you can register for Disney's Disability Access Service (DAS) pass. Once you have the pass, when you go to queue for an attraction, you'll instead be provided with a return time consistent with the current wait, and you'll come back at that time to ride the attraction with minimal wait.
You can (and should) pre-register for your appointment to determine your eligibility for DAS. This advance appointment needs to be between 2 and 30 days of your trip. This will save you time at the parks, and avoid any surprises if you're not deemed eligible.
You can get more details and make an appointment to pre-register here.
Notably, with a few exceptions, most queues are wide enough to accommodate wheelchairs and electric scooters (aka Electric Conveyance Vehicles or ECVs), so people with simple mobility issues are not typically eligible for DAS. If that's the issue, you'll be able to tour the parks with minimal disruptions without any special advance planning; when arriving at the attraction, just ask the cast member at the entrance if you need to do anything special.
Wheelchair/ECV Rental
If you need a wheelchair or ECV at Disney and don't have your own, you basically have two options: renting one at Disney, or renting one from a third-party. There are pros and cons to both.
If you rent from Disney, you will pick it up at the entrance to the park and turn it in when you leave. ECVs currently cost $50/day (wheelchairs are $12), and if you park hop, your receipt will allow you to pick up another at the new park. On the upside, not having to manage an ECV gives you more Disney transportation options and is a lot easier if you have your own car. On the downside, you only have access to it in the parks, and will have to walk anywhere else in the resort. Also, the Disney ECVs are quite slow and clunky. Finally, they can run out -- this is less of a problem if you are there early, but can present an issue in the afternoon, when park hopping during busy times.
If you rent from a third-party, you'll typically have your ECV delivered to your resort and will have access to it wherever you might need it during your trip, but you'll also need to find space for it in or near your room. Preferred providers (ScooterBug is the only such provider as this goes to press) will handle pick-up and drop-off through bell services and you need not be present; all other providers will require you to be present to meet the vendor for pick-up and drop-off. Third-party vendors tend to be cheaper, with the non-preferred providers being cheaper than ScooterBug.
We personally prefer using a third-party vendor when we have needed an ECV, but it really is the sort of thing that is a matter of what's a better fit for your situation.