I can hardly contain myself when I see someone treat someone else so rudely...I think I would have to say something.

Of course, then I would be rude. haha I have not personally ever seen or experienced anything like what you guys are describing, Thank Goodness. I have only been to WDW once. There certianly are plenty of obnoxious people to go around, though.
I really feel our entire society is lacking manners. When I was growing up manners/ettiquite were taught at home and at school. These days...it doesn't seem to be taught anywhere.
My children are DS12 and DD7. I personally ask my children's teachers to please make sure my child is always polite and respectful. We insist on thank yous, please, yes mam and no sir at our house. We certainly are not stick in the mud people either.
I cannot tell you the number of times that people (usually complete strangers) comment on my children and their manners. My DS open doors and lets girls enter buildings before him and you know what the girls love it

.
I really feel the lack of respect for others is just a reflection to where our society and world is these days. It is really such a shame. I tell my children I do not care what anyone else says or does you will be polite and respectful. They don't argue.