We have been in WDW in Jan and Mar of this year and Feb of last year.
January:
cheapest rates, coldest weather, parks close early, some rides are closed for refurb
crowds - mostly low - as long as you avoid the MLK crowd and tour groups
February - early in the month will still have low rates, the weather tends to be a little warmer, it is the driest month of the year in Florida, and the crowds are still low as long as you avoid President's Week. There will still be rides closed for refurb but the parks are starting to extend their hours a bit.
March - rates are higher, weather is warmer, crowds are greater, most rides are open, park hours are extended and the Flower and Garden Show opens in March. This is spring break time, but the weather is great.