1. Rides, attractions: This is, after all, the reason I pay for entrance to the parks. If it weren't for artistic and technological monoliths like Test Track and Space Mountain I'd just lounge around the pool for whole time I was on property. I just visited WDW two weeks ago, and that had been the first time since 1999. They had to open Space, Cretacious Spin, and Tower of Terror 4 to get me back. If they don't open any new big rides, I can live with Six Flags.
2. Cleanliness: The expectation at Disney parks is already set so high that any kind of filth or disrepair really stands out in your mind. If they hadn't already put so much time and money into the way things look it wouldn't be such a big deal, but to be so close to perfection you find that small things like chewing gum stuck to trees and chipping paint on buildings stands out all the more.
3. Service: I actually tend not to have much interaction with the cast members when I'm there. Like the cleanliness issue, they never really stick out in my mind until they actively interfere with my visit. But that's just me. I like machines better than people anyway.