Here's a link to historical weather data, temps and rainfall:
http://www.srh.noaa.gov/mlb/mlbclimat.html
We've been there at least 7 times in the first 10 days of March over the last 10 years, spending from 7 to 11 days at a time. In all that time, we've had one day of steady rain and half of another day, not on the same trip, other wise we've had mostly sunny weather in the 70s and 80s. It's definitely pool/waterpark weather. In 2005 I understand Orlando had a cold snap during that time, but we had to switch to Jan. 1 - 10 that year due to the impending birth of a grandchild so we missed that. On all my March trips I've never used long pants, even at night although a few times I wore long sleeve shirts to PI at night.
The crowds are 'Moderate'. Get to the parks at rope drop, use FastPass and you'll do fine. We follow that plan then hop to Epcot every day when our early park gets stand-by lines longer than 20 mins. WS rarely seems crowded to us. It will be ESPN Weekend at MGM February 29-March 2. That park will have sports related broadcasts and displays, but I've found that the actual rides are walkons because people not interested in sports avoid MGM those days. The plaza area by the Hat will have a stage with ESPN and sports celebraties but the rest of MGM will be little affected by the event. The ESPN Club at the BoardWalk will also have some stuff going on if my memory is accurate. It's a great time to go, especially since we get to escape a crappy PA Winter for the sub-tropics.
Bill From PA