I was at Epcot today and it rained starting at lunch until close. We loved that it wasn't very busy outside because of the rain. The only downside was that they closed Test track even before it started to rain. I agree with the extra shoes or flip flops. Also, try to get the ponchos that are a little heavier. My family had two types, 1 for $1 and 2 for $1. The one for $1 each was better...sort of like comparing generic garbage bags to a brand name one. If you have little ones and using a park stroller, bring a see through plastic table cloth and binder clips to help keep it in place. We had a lot of people comment and ask us if we purchased it in the park.
I do wish we brought some kind of towel, like a shamwow, to wipe down seats on rides and then wring it out. Some rides allow riders to keep ponchos on, so when the riders keep them on for the ride, the seat gets wet.