We were at Disneyland in the first week of December 2011 and found the crowds were nowhere near as bad as I'd heard. We hardly used FPs or queued for anything. There was a need for FPs at World of Color for close to the water, which we picked up as soon as we got into the park.
We found the mornings and late afternoons chilly (but we are nth Qld'ers) with a need for jackets, gloves etc. We stayed until after the fireworks every night and on some nights we found it really really cold by that time. During the middle of the day, though, it was lovely. It did seem to start getting dark around 4.30pm and that's when we felt it get chilly. Remember to bring layers and you can always hire a locker to store stuff if you need to.
We were blessed with no rain but the fireworks were cancelled one night because of the strong Santa Ana winds, so keep in mind that the decision to cancel them is made at the VERY last minute and not made lightly. We did have the snow to compensate. I didn't care as it was my first time at Disney and I was just so excited to be there in front of Sleeping Beauty's castle regardless.
Enjoy your trip. Remember to take time out to just sit and watch and listen. And get a book on Hidden Mickey's from a gift shop as soon as you get there and follow the trail. I didn't get mine until we were leaving on the second last day and I think I missed out on so much because of it.