My main rule for WDW? Keep in mind that its a vacation. Take things, yourself included, a little less seriously than at home. Be a little nicer, a little more easy going, a little more forgiving. Have a great time, but remember that most of the others there are on vacation too, so try not to interfere with their good time, either. Remember the magic. Listen to what the kids have to say, rather than just dragging them along to a series of attractions. Sometimes the ladybug on the leaf at MK is much more enjoying to them than Space Mountain. Under all circimstances, do not become the parent screaming at their kid that they did not pay all of this money to bring them here for them to want to sit down and rest. In general, don't sweat the small stuff, and pick your battles carefully.
One that I dont think has been mentioned yet- don't push and shove trying to get a seat for the shows. The cm's allow only as many people in the preshow area as there are seats, so there will be a seat for everyone, and they all offer a good view.
Six people do not need to stand in a crowded line together for one hot dog.
I agree that people need to watch their language! I'm guilty of slipping up at times, but its nice to at least try. Along those lines, the line for Small World isn't the best place to discuss the plot of last night's episode of "Sex & the City" either.
Do some research before your trip, even if its very basic. This is one of those "for your own good" rules, rather than a "to not annoy others". How many times have I heard someone at Epcot asking how to get to Space Mountain, or worse yet, how to get to Disney World!
These two are resort related rather than park related, but I think they still deserve a mention-
Please do not let your children run down the resort hallways screaming, especially early in the morning. The walls aren't all that thick, and after long WDW days, we need sleep!
In resorts with balconies, *especially* DAK-Lodge, please keep in mind that you are not the only one trying to enjoy the view! One morning while I was trying to enjoy a peaceful breakfast watching the animals, the family a few doors down from me was seeing who could make the loudest duck noises.
Common sense, "please" and "Thank you", a smile, a good attitude, and sunscreen are your friends.