I went to Disney World last summer at age 21, my first trip was age 7. There were some rides that I couldn't go on that I thought I could. I also have some viewing/ride access tips.
When the map says
ECVs must transfer-it is talking about the 3 wheeled scooters or the chairs Disney rents NOT electric wheelchairs. Here is a list of rides that you can drive your wheelchair directly onto without transferring, sorted by park:
Epcot
Ellen's Energy Adventure: Only manual chairs allowed
Turtle Talk with Crush:
There is a separate wheelchair line. You might be given a return time. A girl in a wheelchair was allowed to sit up front with the kids (towards the back) but the parent had to ask.
Soarin' (transfer, but...):
Cast Members request that you park by the pole and walk (or are carried) the few feet to your seat. They will allow you to park right next to the seat and move your chair if you request it. Since I have breathing problems, I pulled the seatbelt all the way out looped it around the armrest (a tip I got from the Pooh sized thread). I was completely secure. The second time I rode a cast member made me unloop the seatbelt but I left it slack and was fine. I also used the strap between the legs for extra security. Grandma held my right arm and Dad was on my left with an arm in front and an arm in back. I was very secure-no problems-and I cannot sit up in any chair or surface (besides my wheelchair) without being held up. DO NOT ask if you can have a family member support you on the ride (made this mistake at DL).
Living with the Land:
Two wheelchair boats with one spot each. No FP needed-separate wheelchair entrance. You may be given a return time. We had a slight problem being allowed on this ride. At first the cast member said they only allowed Hoveraround chairs (???) but after my mom explained that experienced drivers of electric wheelchairs can make very tight turns, he let us on. You may have problems depending on the CM. Those who have problems using steps can walk down the ramp in the wheelchair boat.
Journey Into Your Imagination with Figment:
2 wheelchair cars with one spot each. The easiest wheelchair slot. Drive forward, the CM twists a platform and you face forward. At the end, the CM twists the platform and you drive off facing forward down a ramp. My chair was a tight fit. I took off my footplates. At the end, there is an effect that may cause a startle reflex.
Honey I Shrunk the Audience:
Separate wheelchair entrance-no FP needed. You will probably be given a return time. Unless you transfer to a seat, you will not get all the effects.
El Rio Del Tiempo-NEW name ??? 3 Caballeros (Mexico):
2 wheelchair boats-one spot per boat. The seats are pretty low if you transfer. There is a separate wheelchair entrance. But if you have trouble walking do NOT park your chair outside because you will not be allowed in the wheelchair entrance. We got separated from my Grandma this way. She was fine but walking the cue would have been bad for mom because the line is on a sloped surface. There is space to park about 4 chairs by the ride and you can transfer from that point.
The American Adventure:
You will be directed to seating in the very back unless you request the handful of seats up front (we sat in the back)
Illuminations:
Several of the wheelchair seating areas have trees blocking the view. The seating area by the Refreshment Port and by bathrooms (including Companion) does not and has a great view.
Epcot has good Companion Restrooms. You can get a list of the locations at allearsnet.com and First Aid at the park
ALL SHOWS HAVE WHEELCHAIR SEATS- The World Showcase is accessible (*note: The Kid cot fun stop crafts will be difficult to do unless you have a tray on your wheelchair because the table and chairs are low and the supplies are scattered)
Magic Kingdom
Tomorrowland
Buzz Lightyears Space Ranger Spin:
I did not fit. My midline control was in the way. My sister almost fit but not quite. She might have fit without her desk. If it's a standard wheelchair he should fit...
Stitchs Great Escape:
Only manual chairs are allowed on this ride. The batteries and different style backs prevent the electric wheelchair from getting into the proper position.
Carousel of Progress
Wheelchairs are parked on a slope.
Fantasyland
its a small world:
2 wheelchair boats with spots each. Enter through the exit.
Mickeys Philharmagic:

!! The wheelchair spots experience all the special effects!
Many Adventures of Winnie the Pooh
1 wheelchair car with one spot. I thought backing in was easier. Request the bounce button be switched on for more affects.
Adventureland
Magic Carpets of Aladdin
Only accommodates manual wheelchairs. The device that secures the wheelchair goes between the front and back wheels and is about 6 inches tall. Batteries get in the way.
Jungle Cruise has a wheelchair accessible boat that you must request
Accessible stations for the railroad are Main Street, Mickey's Toontown, and Frontierland. Two wheelchair spots.
Liberty Belle Riverboat
Spectromagic & Wishes viewing tip:
If you plan to watch both use the Spectromagic wheelchair viewing area by the partner statue. Then you are situated for a great view of Wishes-just turn around. They do make two rows of wheelchairs for the parade...
ALL SHOWS HAVE WHEELCHAIR SEATS
Disney's MGM Studios
Star Tours:
There is a nonmotion ride available! I was thrilled to know this. There is a separate wheelchair entrance so no FP is needed. Nonmotion option is only available in the evening
Indiana Jones Epic Stunt Spectacular:
Ask to sit up front for a better view...stay after to meet Indiana Jones.
Backlot Tour:
2 wheelchair spots...I don't know how many accessible trams there are. Mom walked up the ramp to transfer.
Journey Into Narnia:
3 or 4 wheelchair spots-depending on the CM. We fit four (barely).
The Great Movie Ride
2 wheelchair trams with one spot each. I took off my footplates and extra bags and fit (barely) by backing in.
Beauty and the Beast:
Ask to sit up front for an excellent view. Those in ECVs may be asked to transfer.
Tower of Terror:
If you can't ride, you can still watch the pre-show.
Fantasmic!
CMs will direct you to where to sit (you can't pick). Wheelchair spots are marked and have a comfortable amount of space between them.
I had a hard time locating the Companion bathrooms. One of the best bathrooms (female) is the one located just outside the entrance to the park. The stall is as big as a companion restroom and has a sink. It's usually empty or has very few people.
ALL SHOWS ARE WHEELCHAIR ACCESSIBLE AND THEY ARE AWESOME
Animal Kingdom
Kilimanjaro Safaris:
2 wheelchair vehicles with one spot each. I find it easier to back in. Usually has an extra long wait for the wheelchair car
Festival of the Lion King:
This show has some of the best wheelchair seats. I also happened to be at the sign language shows. I found my self watching the interpreters because they were so good. There expressions and body language showed the emotion of the music...
Remember that all shows in all the theme parks have wheelchair seating.
My recently completed trip report can give you an in depth view of the good and bad...
Good luck!
Christamae