The week between Christmas and New Years and the two weeks surrounding Easter are the busiest all year at WDW. You'll be experiencing wall to wall crowds. It can still be a fun trip, but you won't be able to do everything. Make a "must do" list, get to the parks early, and utilize FP as much as possible.
One plus? The weather should be really nice- not too hot, and the humidity of summer hasn't kicked into high gear yet.