Thanksgiving week is good until Thanksgiving Day. Thursday-Saturday get way crowded. Friends of mine have gone the week after and experienced low crowds. March (outside of Spring Break) is good too. The only thing you give up is the extended hours.
The Christmas decorations take the Disney magic to a whole new level.
On a side note, Disney Cruise Line is great! Cruised with them in January and it was the best cruise I've been on. Typical Disney high quality.