typically after Christmas and new years week the place is a waste land.. you will be able to ride every ride twice at least in a day.. there will be no line to most stuff and just waiting for the next roller coaster cars to come in to loading station.
great time if you want to get a bunch of riding in but kinda boring with out all the other humans around to get in the way and look at.
As a reference, in the past, Islands of Adventure has hit capacity for the first time in HISTORY the winter after Potter first opened. December 27-29 were the worst days. This held true the past year as well, when on December 28 every single theme park in central florida hit capacity in the same morning (even Legoland). I cannot stress enough how much of a horrible time to visit this is. As someone who works at Central FL theme parks and genuinely wants guests to have a good experience - please come earlier or later. You will be miserable otherwise.
Walt Disney World's Magic Kingdom
Disney's Animal Kingdom
Disney's Hollywood Studios
Disney California Adventure
Universal's Islands of Adventure
Universal Studios Florida
Universal Studios Hollywood