My family and I just returned from a week at WDW and I have to agree with the majority of posters. An afternoon shower can be a welcome event, especially when it cools down the temperature and thins out the crowds. The negatives are when there is lightning in the area, which can close rides and attractions. We were at HS on 7/12 and the Indiana Jones show was canceled because of lightning, which I would not have even considered lightning as a threat to that attraction. The rains on 7/12 kept the pools and rides pools closed most of the night.
The rain on Monday (7/17) was unlike anything I have seen in years. It was a constant, hard downpour. We planned on going to MK during the evening but decided against it. These were the only two days where the rain really negatively effected our plans.
We were at WDW during Tropical storm Debbie in 2012, but it didn't effect us too much. The rain kept the parks empty and the temps down; there wasn't a lot of lightning, so rides operated and the pools were open.