I can't remember ever going to WDW that it didn't rain at least 25% of our days. Most of the time the rain is over in an hour or two. Our last trip, Nov/Dec 2005 we had 3 days that it rained all day and all night, that's not counting the days it rained just for an hour or two. But the rain has never slowed us down.
Yes, they have cancelled the Osborne Lights, the fireworks displays at all three parks, the castle shows... but there is always so many things to see and do that if one is cancelled just go to another.