I was in WDW the second last week in August for two weeks, and it was an English holiday but not an American one, the parks were not busy - Big Thunder Mountain was the only queue over 45 minutes the entire stay. Probably the best time of year I've went as well.
I've never stayed in a Disney Hotel due to the huge price but from looking online the saving on the dining plan is almost worth it, eating in Disney is pretty expensive if your not going to counter service so would definitely probably have a dining plan on my next visit.
Extra Magic Hours unless your there the whole day seem to properly ruin a park unless its the Magic Kingdom, for example on our first day we went to Animal Kingdom it had early morning magic hours and the place was struggling with the number of guests, at first I thought I had booked a terrible time to be in Disney but then day two at Hollywood Studios was surprisingly quiet and this continued the rest of the time.
Weather changes rather quick, but its still too hot
I was never a fan of SeaWorld but prefer Universal and Islands of Adventure, Universal looks as though its been through a lot of updating too! A day at each is a must