I'm never disappointed in my trips to WDW. I get disappointed with THINGS that happen during my trip (seeing Splash in such a sad state disappointed me, the food at a certain restaurant disappointed me, not seeing the headless horseman ride before the Halloween parade disappointed me...) but the trip as a whole was not disappointing.
I think most people (I know I am) are disappointed with the fact Disney can do so much more to help spruce up the parks and it's frustrating because they don't. It seems though with the FLE and Disney Springs on the way that they are taking a step in the right direction, but this step is LONG overdue.
They increase park tickets yet don't give you new attractions and hardly keep maintaining the old ones (the Laughing Place on Splash anyone?) they cut some meet and greets, keep an old parade.. They increase dining prices while the food quality declines.. They raise resort prices yet cut EMH, a perk to staying on property. It just to me feels sometimes that with all these increases, where is all the money going? They keep cutting things to make more profit, but they profit should be used to keep creating more magic. Obviously they are spending to expand MK and create Disney Springs, but that's obviously not ALL of Disney's profits. That's only a fraction of them.
Disney is not the run down traveling carnival some people make it out to be. The parks just don't look as awesome as they used to and I think that is what frustrates people the most. But as far as coming away disappointed, you won't. Not unless you go in looking for everything that's wrong.