My of these have already been posted, but it's easier to re-list them than try to quote them all and agree.
-People who stop in the middle of a doorway/exit path/ride entrance/crowded street. Whether it is for a photo, to look a the map, or to open a stroller, it is annoying. Pull off to the side.
-People who smoke in undesignated areas. No matter how discrete you think are trying to be, you can typically smell it within a a large distance around you.
-Lazy people using ECVs. I say lazy, because not all ECV users are fat and not all fat people use ECVs. Walking is good for you, even if it takes you longer and your feet hurt from doing it. Try walking 10miles a day everyday!
-People who change diapers where they please. The restrooms have changing tables. There is no need to change a diaper on a park bench, restaurant table, or trash can.
-People who are too loud during a show or attraction. For the courtesy of others, just enjoy the attraction or show quietly. No one wants you to sing along, quote the ride, talk on your cell phone, yell at your kids, make commentary, etc.
-People who don't clean up their trash. Disney places trash cans EVERYWHERE it seems. Is it really that hard?
-Parents who demand their child be happy/have fun/or hurry up in the restroom. How would you like it if someone told you to be happy/have fun/ or ______ faster?
-Children with no basic manners / Spoiled Brats. Excuse me, please, and thank you are the basics. These are often times the only thing missing from a great child to a spoiled brat.
-Parents who put their children on leashes. Yes, it is easier to keep up with them, but it looks tacky and shouts "Hey, I have a hard time keeping up with my own kid."
-People who take flash photos when they told not to. Even if you don't know English, I have the common sense to think about it or can easily get the message after you do it once or twice.
-People who make a mess in the bathroom. These are the people who complain about messy bathrooms, so why do they do it. Things like flinging your wet hands all over the place or leaving your paper towels on the floor.
-People who stand in line for food and don't know what to order when they get to the counter. If you stand in a line in the first place, you should have the time to figure out what you would like.
And to sum things up and cover every other one...People who are oblivious and don't think. The guy who has a balloon and doesn't lower it during the fireworks show and blocks the view of those behind him. The person who takes their time using something not realizing the line of people waiting to use it. The people who only focus on getting their party and their strollers to the next destination and not on the general flow of traffic. The guy that ignores the rules/instructions because they don't think they apply to them that one time.
People just don't think things through...how their actions affect others, how to be polite in the presence of others, and how to take other people's wishes into account.