Last year, it rained 6 of the 10 days we were at WDW-on two of those days it rained all day. Here is what I learned:
1. Even if it is summer, wear something warmer than shorts and a tank top. If your skin is wet, you will freeze when you go into an air conditioned store or retaurant.
2. Those $1 ponchos are miserable; in fact, all ponchos are miserable with kids, because they take them off inside out and the when they put them on the wet side is in, etc. Take light weight rain coats and ball caps.
3. MK is great in the rain. There are so many rides and attractions so close together. One rainly morning we were the first ones to ride Dumbo (of course, our feet were in several inches of water). We ran around and rode all of the Fantasyland rides that we usually don't have time for, because everything was walk-on.
4. AK is miserable in the rain; almost everything is outside.
5. I wasn't fond of Epcot WS in the rain because most of the entertainment is outdoors and that was cancelled.
6. MGM is okay in the rain.
7. If you wear dark ponchos, take them off for character photos.
Have fun!