It's been so interesting to see the crowd patterns shift over the last decade. Josh talks about this a lot on his site. When we first started taking family trips in the 2000's the summer was insanely crowded while the early fall and January through March were the best times to visit as the crowds thinned out and the temps cooled off.
It seems like Disney's efforts to even out the crowds year round combined with peoples desire to visit when it wasn't so hot has created a huge change. Now, many times in the summer the parks are dead and the old "best times to visit" are now mega crowded. WDW has basically become a year round destination where certain holidays and events drive crowd size much more than the seasonal calendar.
