Someone else had posted that over at touringplans, a lot of their crowd size predictions have shifted significantly lower. I've noticed it too. Not 100% sure why that is. I've read some articles about how, even though average incomes have gone up and unemployment is lower, actual purchasing power has gone down. I wonder if it's enough to discourage travel...
There's also been a bit of a slump amongst foreigners coming to visit America the past few years; I'm sure that affects smaller big cities more than the biggest tourist destinations like NYC, LA/San Francisco and Orlando, but that doesn't necessarily mean they're not affected at all. But if fewer people are coming in to visit, and fewer Americans can afford to go away on vacation...
I also suspect the affect on the economy, combined with higher gas prices and Disney's continued habit of hiking prices higher than average cost of living increases, and you're seeing fewer repeat visits by repeat customers. As it's more expensive to travel, maybe people who'd normally visit multiple times a year are opting for one longer trip.
All of this is just me spouting, take it with a grain of salt.