We were at WDW last year over Memorial Day. We came down the week before, had the long weekend and left later that week. The crowds the week before were great. Decent lines and good fastpass times (1-2 hours from time we pulled them.) By the weekend things changed. The parks did get crowded. We avoided MK on Sat., Sun., and MemDay. But we always avoid it on weekends. Also, Star Wars weekends may have been going on--I know we avoided MGM for some reason. We spent most of the long weekend at Epcot (our fav.) and wandering around the Boardwalk. Even with the crowds we were able to do most of the things we wanted to--but we had 12 days and took it slow. The best thing was that we were at the Swan for the last of our stay. Memorial Day saw few conferences that week. We got a great rate and the hotel was not crowded.
Weather--well, it was hot. In the 90s most days and humid. Drink lots of water and stay hydrated. Take breaks during the hottest of the afternoon and the worst of the crowd. AND have FUN!
-Jockaroo