Well, I'm going to be realistic and honest and say that you
will run into large crowds. Not New Year's Eve-level crowds, most likely, and not as bad as the crowds in between Christmas and New Year's, but there will be a lot of people. The reason I am telling you this is so you'll be mentally prepared for it. It's much better to expect the worst before you go and then either be pleasantly surprised or at least be able to adapt because you were prepared.
I agree with the PP's - think positively, but based on my experience last year, I cannot tell you that it won't be very, very crowded because it just might be. It might
not be, but better to plan for crowds than to go in not thinking it will be too bad.
I was there last year on December 19th - a weekend - and it was more crowded than I had seen it in
years, and that was despite the SoCal AP holders being blocked - which is proof positive that it is
not just local AP holders contributing to massive crowds. The entire 2009 holiday season was the busiest holiday season at DLR in a decade, according to reports that came out later.
The weekend before Christmas is always very busy, but I went on that same exact weekend in 2007, 2008 and 2009 - and the 2009 day was, by far, the most packed of the 3 trips. Haunted Mansion Holiday's line was 45 minutes long before 11:00 a.m. DL reached capacity early in the day. The POTC line was the longest I had seen it in almost 20 years. The back alleys of NOS were almost impossible to walk through. The area in front of IASW Holiday at night was an impenetrable wall of people, and when we saw it we turned and walked away!
I was not prepared for that level of crowds because it had not been like that on my 2007 or 2008 trips. It was very daunting to me and I couldn't get all the things done that I wanted to do because I was too frustrated and there was just not enough time to "come back and do it later" for certain things.
This year, with World of Color thrown into the mix - which has increased attendance in both parks this year - I cannot imagine that it will be any less crowded. Hopefully it will be, but I wouldn't expect it.
So the very best thing you can do is just get yourself in that mindset of thinking that it will be very, very crowded. Don't let it stop you, but just expect it and plan for it. Think positively and plan wisely.

Use all the tricks and time maximization techniques that you can and have back-up plans in place. Make lists of priorities and secondary priorities. You will still have a
great time - I did - but I don't want you to show up, expecting a certain crowd level, and then being in for a shock!!