Welcome to the boards.
I teach in Canada and have to take Disney World trips during our Spring Break, August or at Christmas. All I've ever known is going during peak crowds, so that's all I can comment on. Our first trip, in 2003, was in August, and although it was awesome, my family found it a bit too hot and humid (and believe it or not, we weren't escaping cold Canadian weather because our weather at home in August was hot and humid as well). Our trips since then have been at Christmas and it is a great time to go! The place is beautifully decorated, the weather is comfortable (especially if you're from the north) and the parks are open really late.
Might I suggest a couple of tips to help with crowds. Go to the parks early, relax at your resort or do recreation (golf, etc.) in the afternoon and hit the parks at night until close. I'm sure you've heard this before, but it helps a lot. Let me illustrate by way of example by comparing 2 days from our Christmas 2005 trip.
On Tuesday, we arrived at MK at 11am, and the place was packed. Fastpasses were nearly gone for most attractions and the lines for the rides were long. We spent the afternoon at the resort and chose to relax, swim, read, etc. In the evening, we had supper at 7pm and shopped for a bit before Spectro Magic and Holiday Wishes fireworks. AFTER THE FIREWORKS, while many people, especially those with children, headed for the exits, we rode Splash, Big Thunder, Splash and Big Thunder with no wait.
The next morning, we arrived at MK at 8:40am. When the park opeded, we got Space Montain fastpasses and between 9am and 11am, we rode Peter Pan's Flight, Small World, Haunted Mansion, Splash, Big Thunder, Pirates and Space Mountains with the longest wait being 5-10 min.!! We had lunch in the castle at Cinderella's Royal Table, then shopped and watched the parade. We did lots between 9am and 3pm and were quite content to go to the resort.
Most everyone INTENDS to go early, but most arrive at about 11am. Go early, go late...but by all means go at Christmas. It rocks!
BTW, we always go around Dec. 27th, and stay as long as we can in Jan. The crowds do thin some and 'value season' at the resorts starts Jan 1st.
Enjoy!