I was in WDW thw first week in March earlier this year. I left swearing I'll NEVER go back in the summer ever again! We were there 3/5-3/12, and never waited on line more than 20 minutes. We stated at the ASMovies and the buses were great also, never waited too long. We didn't use Fast Pass until our last day in the parks- 3/11- when kids from a cheerleading competition arrived. Even then, we just used it once in Epcot for Test Track.
I was there in March a few years ago with an ex- a trip I have blocked out of my mind-that time we experienced similar crowds, but the weather was COLD. This year the weather was perfect. A little chilly in the early morning and evenings, so I always had a sweatshirt around my waist. I wore capri pants and t shirts all week, and my DH wore shorts a few times. It only rained once- one entire day- and it was cold, upper 50s. But I would say the daytime temps were generally mid 70's to low 80s. It was perfect!