I agree with the previous posts. While I'm sure we're all sympathetic to the needs and challenges of individuals dealing with their various disabilities, that doesn't mean they can't be accommodated in a more reasonable manner.
Being in a wheelchair does not mean you can't wait in line just like everyone else. Find a way to "mark" their proper place in the queue (by having other members of their party enter the line or some other means) and when their time comes up, let them on. I just wish ride designers could come up with ways for them to board the rides like HM, SSE etc. without stopping the entire ride, thereby taking away from the experience for everyone else.
As far as ADD or ADHD, my opinion is that no exception should be made at all, not even with a Dr's note.
There are some individuals who are obviously seriously disabled, especially children, and for them clearly there should be some reasonable accommodation made, but there are many, many individuals every day who are taking advantage.
I'm curious about the guy yelling because he couldn't get the handicap stall. Was this guy in a wheelchair? Because you didn't really make that clear. Obviously, the handicap stall is for individuals who are in wheelchairs which won't fit in regular stalls. If he wasn't in a wheelchair, he didn't need it. If he was, then yes, when it next became available, I would have let him use it. I use the handicap stall if no others are available, but only if there is no one there that would need it at that moment.