Just my two cents, but after perusing this thread...it appears to me that rude and ignorant people come to WDW with and without children. The fact is many have cited rude behavior on both parts. And they are right. I have seen many parents do ignorant inconsiderate things in all walks of life. I also have witnessed some very rude adults chose to act incredibly irrational and inappropriate to children.
Here is what I have learned going from a single young adult to a married with children park goer...
1. All children that appear to be "misbehaving" in WDW are not evil! I remember seeing kids act up (pre kids) and thinking, "I would never let my kids do that." LESSON LEARNED: WDW and all very overstimulating activities tend to bring out the best and worst moments. I don't let my son do horrible things or hurt/annoy others, and he is always required to apologize if he has interfered with someone else's enjoyment, but I can say I pick my battles! And we sometimes discuss behavior, post episode, back at the hotel.
2. Parents are sometimes so focused on kids that they zone out. I am not making excuses...well kind of...but I want to let you know, I am a nice and kind person, and sometimes I don't realize I stepped in front of you, or short stopped on you. Again, I would always say I am sorry, but if I was doing something that I didn't realize was infringing on your good time. Just ask, I always try to be considerate, but we all make mistakes.
3. Kids are not little adults. They are learning the ins and outs of good behavior and caring about more than just there own needs. Adults have 20-80+ years of practice. Give the little ones a little slack. They are not sleeping in their beds(maybe for the first time), they are in a overstimulating environment, and they are off their schedules.
That turned into a bit of a novel, but to respond to the original issue. I don't have an issue with people putting kids on their shoulders, if they chose to do it in the back of the crowd with no one behind them. To do it with someone standing right behind you is inconsiderate. However, I would probably just adjust if it happened to me and not say anything.
I am always surprised at the degree of unhappy people at WDW. It's the happiest place on earth!