Gina (rwbeagles) has a Disney World thread going on that I've typed a ton of stuff for -- essentially those are the two busiest months going. All the "snowbirds" come down in January, and July is ... well it's "summer" for the rest of the US but in Floorrr-da it's THE hottest month ... only August is worse.
Both are THE most expensive times to visit and the most crowded.
October (particularly early October) is a dream -- less crowded and far cheaper. Late April is probably the next best time expense wise (get at least two weeks PAST Easter to miss the Spring-break rise in prices.)
By mid-May it gets so unbelievably hot all the locals disappear "inside" until about the end of October. (no joking -- there's virtually nothing going on locally because it's too hot to be outside).
April is also THE best time of the year to get a horrific sunburn -- we're closest to the sun then, and no kidding -- wear sunscreen. It's not 'hot' yet but you're burned before you knew you were hot.
Find Gina's thread -- it's pages long but has TONS of good info from a bunch of us.