We've done that time of year four times (2008, 2011, 2012, 2013). Weather, while changeable and subject to wide variations one day to the next, is great IMO. Generally not so hot that it saps your strength and you just want to get into air-conditioned comfort and not so cold that you freeze standing in line. You do need to be prepared for that wide variation of temperatures and for after dark temperatures. I never wear shorts even though occasionally it can get close to 80 degrees simply because after dark you will want long pants and I don't want to have to go and change. A fleece jacket or hoodie will normally get you through the evenings with a long short underneath; you can take the jacket off and tie it around your waist if it's really warm but I find that you'll still need that long sleeve. Pools or water parks are only going to be good during daylight hours (it still gets too cold after dark for water) and you don't really know in advance if that will be too cold. We always avoid pools and water parks in December. That is one con especially if you're staying at the Beach Club. We've never been able to take advantage of the cool pool there. We're always in the parks during the day and it's definitely too cold at night to swim.
We've always been able to get the hotels we want, ADRs we want and most attractions have much lower lines than at other times of year. Some still get quite busy, which is where we use FP+. For the last few years I've read people saying that the first/second week of December is busier than in previous years, but I didn't find that to be our experience last year or 2012 and 2011. Perhaps their expectations are different than ours or their previous experiences in other years were better but we thought that the time of year works great and will be our go-to time (in the process of considering Dec 2015). Perhaps its just that we compare it to our previous times when we used to go April/May.
Decorations are up, Osborne lights are up, different street performances some of which are focused on the holidays and some of the resorts have wonderful gingerbread constructions to see. Special or different fireworks displays and parades. We never bother with the candlelight processional - not our kind of thing so I can't say much about it other than to make sure you know when it starts in case you have to pass through that area during that time because it often gets blocked off. We've had to find an alternate route a couple of times to get to ADRs on time.
A friend of ours talked us into
MVMCP in 2011 which we previous considered to be for children only (it's just the two of us). We've done it three times now and really enjoy it. We aren't parade people but we do love Disney parades - somehow they are able to make me like them and the MVMCP one is great. The fireworks are spectacular and then we spend the rest of the evening hitting the attractions as the attendance starts to drop off. The hot chocolate and cookies may not be five-star quality but it does somehow make it feel more like Christmas and presents an interesting contrast for us to the warmer weather (even when it's "cold" in Florida, it's considerably warmer than here).
The biggest con for us is that we get back only a week or two before Christmas and I have to start shopping and getting ready for the holidays. I really don't feel like it at that point and have little time to do it along with everyone else who has left it to the last minute. Everyone at work is starting to slow down while I'm trying to catch up and get things done so it's a bit frustrating. Then I have to work through the holidays since I used up my vacation and I can't take time off again even if I have days left and someone has to cover certain activities during the holidays and that's me. However, the pros (slow season, lower cost, greater availabilty of everything, decor, activities, and nice weather) greatly outweigh the cons.