March is Spring break time of rmany kids around the Country, and esp the colleges. So it can be a bit crowded (esp. if Easter falls in March) but otherwise it is my favorite time of year to go. It is also one of the more expensive times of year to go to WDW.
After a long winter, the warm sun feels fabulous! March is comfortably warm enough to go on the water rides and go swimming - if not every day, almost every day, but it is also not so hot that you melt in the heat.
Depending on where you live, you get home just in time to watch the first spring flowers poke their heads out! Psychologically, it's a great time of year to go to WDW.
Allearsnet.com has great info about weather, crowds, etc, at various times of year.