Weather in December is really hit or miss. The key is layers and a raincoat. It's very possible it will be in the 70s during the day, but If the coastal fog works it's way to Anaheim especially at night, you're going to want a very warm coat. It's not uncommon to see people with gloves and hats to help battle the cold at night.
If it does rain, it usually doesn't stick around for more than a day. So you may only need the raincoat for one morning or afternoon - check the forecasts. I don't think El Nino is predicted this year. It has the potential to bring lots of rain.
Expect Mansion and Small World to have large crowds for the holiday overlays. The parks will be busy, but manageable. I'm not sure how the new AP blackout dates will impact the crowds. In the past the parks were very busy as soon as the holiday decorations went up, but then slowed down until the holidays. Since the 10th is the last Saturday for Deluxe APs, I'm not sure if people will be in the park for one last look before the New Year.
Get a park hopper and have a great trip!