Here's how the year breaks down for us:
Jan: Weather is too iffy. Can be downright cold and miserably wet some years. But crowds are great and rates are very low.
Feb: Better weather, but watch out for crowds some weeks.
March: Spring break (and Easter some years). Nuff said.
April: Bingo. Nice weather. Spring break is (mostly) over. Go after Easter.
May: Getting hot. Crowds aren't bad other than over the long weekend.
June/July/August: Schools out. Heavy crowds. Hot. Humid. Expensive. No thanks
September: Still hot but low crowd levels, great rates on resorts. Yes, please.
October: Not as hot. Not as cheap. Relatively low crowds. F&W at Epcot. OK!
November: Cooler. Resort rates are going up and so are crowd levels. Thanksgiving week is super expensive and crowded.
December: Cool. Expensive. Crowds increase dramatically as you get closer to Christmas and remain nuts till after New Years. No way on earth.
That's the way it shakes out for us.