We have been many times early Dec and love it, as its the most festive time at WDW, all parks and resorts are in full decorative mode.
MK has the Mickeys Very Merry Christmas Party, which is a paid event, less people in the park than normal, special parade and fireworks, free hot choc and cookies, special shows and not all, but most rides and attractions open. Even if U choose not to do this, MK is very festive in itself.
Epcot doesnt take a backseat to MK either. Decorated in splendor, narrators from each country dress up, and tell stories relating to their country about Christmas. Not to miss, Candlelight Processional. This is an absolute must see IMO, tells the Christmas story through narration by guest speaker, usually someone from the entertainment world, and a mass choir and orchestra. A good way to get a good seat, do the Candlelight Processional Dining Package, pricey but worth it for a 1st timer to guarantee a seat. It is hugely popular!
HS is dressed up too, be sure to see the Osborne Family Lights on the Streets of America, another not to be missed wonder!
AK is probably the least festive, but they Christmas up the parade a bit.
Other things not to be missed, resorts, these are my faves, take the time to check out, def. worth it. GF, huge tree and life size real gingerbread house, smells really good. Poly, just neat seeing the Hawaii influence in decorations. BW, really cool decs., with gingerbread influences. Last, not least, and my fave among em all, WL. Huge tree, and just amazing to see.
Have fun!!