My last two trips have been in December (Sunday-Thursday), and I've always had great trips.
Pros:
Very quiet during the week, both in the parks and the hotels (I'm now ruined for any other time of year as I think a 15 minute queue is long!)
The Christmassy themeing is lovely
Hotels much cheaper
Cons:
The weather's unreliable (although the cold is reliable - pack thermals and waterproofs!)
Rides may be under repair (although on a four-day trip, there's never been anything closed the whole time)
Not all of the restaurants are open every day
Shows/parades/tours/rides not necessarily as frequent as other times of the year (eg I think they only had one train running per day so the wait times were longer, Pirates sometimes ran two boats at a time rather than 4)
The shorter opening hours could go on the cons list, but I found that as you can fit so much in in a day it didn't really matter.
I'm not sure I'd go at any other time of year any more, unless there was a special event
