It all depends if you are staying on property.....
My DH and I were in WDW last year on the 4th of July. Yes- it was mobbed in the MK on both the 3rd and 4th- however- we still were able to get in around 4:00pm. I'm sure the parks were closed to people staying outside WDW. I can't think of anything much that was extra special. Poeple decorate their golf carts at FW and it's like watching a parade come through. They swore new citizens in the MK in the am. Other than awesome fireworks- that's all I got!