When I go to Disney, I expect a seamlessly magical experience and a higher standard of excellence than I expect from any other vacation destination.
I expect cast members to be friendly (though I understand people have bad days and as long as a CM isn't unacceptably rude I wouldn't complain about it) and to acknowledge birthdays/special events. I expect a clean resort room and if there are any issues with things in my room that they be fixed promptly (broken toilet, tv not working, etc.). I expect for cast members to maintain the integrity of the "show" (my past two trips I've seen cast members walking in lands where they don't belong in costume, like a frontierland CM in tomrrowland, which to me is unacceptable. Walt made utilidors for a reason). I expect high quality attractions and experiences that make the price of admission worth it. And, most of the time, Disney meets my expectations.
For the majority of us, I don't believe we expect too much out of Disney, because the fact is that the expectations that we have (or at least that I have) come from previous experiences. And if we've already experienced it, then it means Disney is capable of meeting, if not exceeding, those expectations. I don't have such high expectations of Universal or SeaWorld, because I've never had such incredible experiences or wonderful guest service at those parks.