First. I highly suggest reading this thread:
http://www.disboards.com/showthread.php?p=16541636
--I would definitely stay at least a week. That way, you have enough time to see the parks and REST as well. They will be crowded, so your feet will need a break. You will be amazed how big MK is vs. DL. There is a lot more space between attractions, so that means a LOT more walking.
--Dining reservations are a must. It isn't like DL. When I called a few months out for PS, I had the pick of the litter. With WDW, you really want to make your ADRs as close to that 180 day mark as possible. I think the food (overall) is better at WDW.
--Stay on property. It will save you more time and the Disney transportation is great. It is also great to have the whole Disney immersion!
--Read comparisons of the rides at WDW vs. DLR. If things are insanely crowded, these are the attractions I would skip **if need be** because either same at DLR or not as good IMHO: Pirates (way shorter), Buzz LY (pretty much the same, but the controls are much better at DL), Toy Story (same aside from building/queue), Muppets 3D, Capt EO, Small World (far cooler at DL...esp outside), and Tiki Room (DLR far better as it is the whole show). I also think the DL Fantasmic is FAR better. Just my 2 cents!
Rides at both DLR/WDW but don't skip: Jungle Cruise (similar yet different), Splash (I like WDW a bit better), HM (diff than DLR a bit), BMTTR (diff a bit), Star Tours (cool queue), and Space Mtn (but DLR still wins).
--Save time to explore outside the parks. DTD is much much larger than the one at DLR. The resorts are also really unique, so it is nice to make dining choices so you have an excuse to see a different resort.
--I strongly suggest, especially since you plan to go there during a busy time, to do a touring plan. I have had a great experience with RideMax, but there are other online services as well. Also, definitely get to a park at Rope Drop (when it opens.)
--Bring some cold weather clothes. December can be either warm, mild, or...cold. You could experience a cold front while at Disney (cool in the evening), so layer!
--And lastly----- look at the map when at MK. Don't assume that it is the same layout as DL. The first time I went, I insisted that we could walk from Thunder Mtn. around the back and into FL. I assumed it was the same layout!! The rivers of America also seemed to be in the 'wrong' place for me!

As DL was my home park, I just didn't get it at first.
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