Years ago, early December used to be a good time to go, before the Pop Warner Super Bowl and the cheerleading competition (Are those still even going on there? It's been a while since I looked). Early January used to be dead before the WDW Marathon took off. February up until President's Day was good but they started the Princess Half-Marathon. Late April thru mid-May was good but now there's the Flower & Garden festival and a RunDi$ney event. September was good, until they expanded Food & Wine and added another RunDi$ney event...
Bottom line - Disney has filled up most of the times that used to be slow with one thing or another - Festivals, competitions, RunDi$ney events... You can get lucky and find some slow days here and there, but the saying, "There are no slow times at WDW anymore" is largely true. It's now just what level of busy you want to endure - Kinda busy, crazy busy, or insanely busy.
BTW - I use "RunDi$ney" because those runs tend to be, on average, 3-4x more expensive than your local events, while giving you less then you get at local events (And please, don't use the "but you get to run thru WDW!!! as a reason for the price. The half and full marathons are at least half on the roads and back-roads within WDW, with no theming and little entertainment, so it ain't all that and a bag of chips). $350 for the marathon at WDW and get a shirt and a usually-cool medal, or $75-$100 locally and get a shirt, a jacket, a medal, and a whole bunch of other stuff, plus you don't have to travel. Hmmm...