Animal Kingdom: I can stay on the scooter for everything except the Safari.
Magic Kingdom: Pirates Of The Caribbean, Space Mountain, Buzz Lightyear & Stitch.
Epcot: Universe of Energy, Spaceship Earth, Nemo, Living With The Land & the boat ride in Mexico.
Disney Hollywood Studios: Toy Story Mania, Rock 'n' Rollercoaster, & TOT.
Disney's information leaves a lot to be desired. They do have an icon that shows a person moving from an
ECV to a wheelchair. In some cases, that means the line is not ECV accessible and the guest will need to switch to a wheelchair for the wait in line.
In other cases, it means that the line is accessible, but there is a ride car which can accomodate a wheelchair, but not an ECV.
And, in some cases, a line is not accessible to the large WDW rental
ECVs and other off-site ones that are that size, but a smaller size ECV would do just fine.
When you get to the parks, pick up a Guidemap for Guests with Disabilities.
These are new and are placed with the other maps when you enter the park.
Look for a bin with a blue wheelchair symbol and the map itself has a black band at the top with the words:
Guide For Guests with Disabilities
Some of what I am writing is just re-interating what others have posted and I will also quote the Guide for Guests with Disabilities Map.
For almost all attractions, the Guidemap says the Mobility Entrance is to use the regular entrance, which they also call the Stand-By-Queue or Standard Entrance.
For attractions with Fastpass, the map says to obtain a Fastpass and if none are available, to enter thru the standard queue or contact a CM.
If you need to enter the line somewhere other than where the map lists, you need to have a Guest Assistance Card that allows for that. In most cases though, you will be able to have the ECV all the way to the boarding area without a GAC.
There is more information about GACs in post #6 of the disABILITIES FAQs thread (you can find the thread near the top of this board or follow my signature to get there).
Animal Kingdom: I can stay on the scooter for everything except the Safari.
All shows are wheelchair and ECV accessible and you can stay on the scooter for until the boarding area of rides to the point of boarding.
The Affection Section at Rafiki's Planet watch does not allow ECVs, but that is for the safety of the animals in the petting area, rather than because of accessibility.
For the Safari you can remain on the scooter thru the entire queue up until the boarding. There is a pull off point to the right side for guests with wheelchairs, ECV, etc that is just after the stroller drop off. This is just before the regular line goes into the regular boarding area. The mobility boarding area has space to park many wheelchairs and ECVs and you can drive it up within a few feet of the ride tram. Someone else in your party can then park it. You will come back to the same spot after the ride. If you can't walk to where your ECV is parked, someone else in your party can get it for you.
There is one wheelchair accessible spot per ride tram. If you are using an ECV and can't get onto the ride car, you will need to transfer to a wheelchair (available at the boarding area) to get onto the tram.
Mobility Access from the map: Obtain a FASTPASS or use the Stand-By-Queue. If Fastpass is not available, enter through the standard queue.
Magic Kingdom: Pirates Of The Caribbean, Space Mountain, Buzz Lightyear & Stitch.
This thread has more information about Buzz Lightyear and Pirates.
Pirates has some access problems because of being an original attraction, built in 1971 when they were not thinking about accessibility.
The boarding area is actually in a different building and down one floor from the unloading area. They were not able to really make it accessible, even with renovation. Because of the difficulty bringing wheelchairs and ECVs in and out(plus one area is tight), they don't allow ECVs in and prefer people to use one of the attraction wheelchairs. Walking to the backstage area is not significantly less steps than just going thru the line. And, as was noted, you do need a CM escort to access that backstage area, so it is not always available.
If you have any problems with getting into and out of low rides, you should be aware that this does have a low seat - below floor level when you get into the boat.
Mobility Access from the map:Listed as transfer from ECV to wheelchair, then Enter through the Standard Queue.
Buzz Lightyear:The Fastpass queue is probably shorter than a city block. See the linked thread for more information.
Mobility Access from the map: Obtain a FASTPASS or use the Stand-By-Queue. If FASTPASS is not available, enter through the standard queue.
Space Mountain: the right side queue is wheelchair accessible and there should be a wheelchair that you can switch to from the ECV (although as pointed out, there may not be a selection of wheelchair sizes).
Mobility Access from the map: Obtain a FASTPASS or see a host for options. If FASTPASS is not available, enter through queue on the right.
Stitch: There is usually not a long line for this attraction and the distance walked is not that far, but before the actual show, you will be standing in several preshow areas for a few minutes at a time - no seats in these. My guess is about 5 minutes in one area, then walk a short distance to another room where you will stand for another 5 or so minutes. The second room has rails that you can hold onto or lean on.
In the actual show area, this attraction has over the shoulder 'restraints' that look like they are to hold you in. They are not actually restraints, but are part of the special effects for the attraction. There are several wheelchair accessible spots where a guest with a wheelchair can park and then pull down the restraint to get the effects the guests in seats are experiencing. An ECV won't fit in these spots.
There will be attraction wheelchairs available to switch to at the entrance and a wheelchair can be taken from entrance all the way thru the attraction and to the exit.
Mobility Access from the map: Obtain a FASTPASS or use the Stand-By-Queue. If FASTPASS is not available, enter through the standard queue. (but, as I mentioned, there is seldom a line for this and you can usually get into the next show whenever you get to the attraction).
I also wanted to mention that Small World used to have guests with mobility disabilities just go through the exit. Since it was renovated a few years ago, the access has been to "Enter through standard queue, then watch for the direction signs to the designated load area." This is on the side of the attraction where the exit is and you go through about 2 turns of the regular line before getting to that point.
If you just go through the exit, you will be going ahead of other guests with wheelchairs/ECVs who are in the line and have not gotten to that point yet.
Epcot: Universe of Energy, Spaceship Earth, Nemo, Living With The Land & the boat ride in Mexico.
Universe of Energy:I have seen people using their ECV in the preshow area and to the point of boarding, but they have wheelchairs available and it would be easier to do that. If you can't, ask the CM at the entrance.
From the preshow area, you will have a short walk to the actual ride vehicle. The distance from the entrance to the preshow area and from the preshow area to the boarding area are all less than a city block, BUT the preshow area has only a few bench seats, so if you do park outside and walk in[, you will not necessarily have a place to sit during the wait for the preshow and the the preshow unless you use one of their wheelchairs.
Mobility Access from the map:Enter through the Standard queue.
Spaceship Earth: I will only add that there is usually a CM checking people in to the area. They kind of line people up in the seating area and board a group or several groups at a time. The ramp to get into the boarding area is a little steep. You can find more information and some pictures on the Epcot FAQs thread near the top of this board.
Nemo: The attraction is listed with the ECV switch to a wheelchair symbol, but I have seen smaller ECVs in line. As was pointed out, the ECV/wheelchair will be moved from the boarding area to the exit while you are riding.
There is a wheelchair accessible clammobile, but it will be an additional wait (last time we rode, we didn't start counting right away while we waited, but I counted over 100 people in line who got on while we were waiting before we were boarded).
Mobility Access from the map: Enter through Standard queue
Living with the Land: To add to what has already been posted, there is a post with pictures about Living with the Land on the Epcot FAQs thread.
There is more information on that thread about getting into the building - you want to bring your ECV in because it is a steep walk up to the building entrance and if you ride Soarin', you want your ECV for that line since it is longer than 1/4 mile.
There is a new wheelchair accessible boat that is not shown on the pictures.
The guidemap says to use Fastpass or enter the standard queue, but in actual fact, guests with wheelchairs are usually boarded thru a separate part of the Fastpass line and there is little wait. The entire queue is visible to you from outside of the ride so you can gauge whether or not you can walk it.
As someone else pointed out, they were able to drive the ECV in line and then straight through to the exit (which is a few feet down from the boarding area)
If your ECV can't go in the line and you feel you can't walk, talk to the CM at the entrance; there are ways they can shorten your walk.
Gran Fiesta Tour in Mexico To get into the building, there is a ramped path toward the Norway side of the building. It is marked with a wheelchair symbol, but we have sometimes found the sign is blocked by people standing in front of it and strollers are often parked there until a CM moves them.
For the ride, there will be a CM at the entrance who will direct you where to go. You can see the entire queue from the entrance. There is a parking area for ECVs for people who feel they can walk the queue. If you can't, discuss your needs with the CM at the entrance. Depending on the size of the ECV, you may be able to drive it in line and just pull through to the exit, then get on. We have seen people do this.
Disney Hollywood Studios: Toy Story Mania, Rock 'n' Rollercoaster, & TOT.
Toy Story Mania: As was pointed out, you enter thru the regular line and then there is different part of the line when the regular line gets to a set of stairs. That is also the Fastpass collection point, so there is a CM always there to direct you.
There is more information about Toy Story Mania (and some pictures) in this thread - skip thru the first few pages because they were discussion before the attraction opened).
Mobility Access from the map: Obtain a FASTPASS or use the Stand-By-Queue. If FASTPASS is not available, enter through the Stand-By-Queue.
Rock 'n' Rollercoaster: WDW has been trialing a different waiting procedure at Rock 'n' Rollercoaser, which involves all guests who are not using FASTPASS getting a card and then waiting together in a larger area until their group is called. Since this is kind of in flux, I would suggest talking to the CM at the entrance to see what to do. It is listed as one where guests need to transfer from an ECV to wheelchair and then transfer to the ride car.