We enjoyed the gingerbread displays (house and carousel) at GF and BC, we also really enjoyed Osborne lights, walking down mainsteet at the MK, seeing the trees at WS in EPCOT. When we were there in 2010 we also enjoyed the characters in their holiday best at camp Minnie-Mickey at AK but I don't know if they are somewhere else with the AK work being done?
Not sure if the Christmas parade will still be playing at MK but its also fun - we love the float with the little train racing around it!
Every park has a large feature tree - the ornaments are perfectly tuned for that park - fun to notice those details...also there have been a few tv specials that show them prepping and decorating for the holiday. I think it's helpful to watch them to get an idea of what you might really want to see if you have to prioritize things!
Happy Travels!!!