January weather is unpredictable. I have had temps in the 50's and temps in the high 80's(sometimes on the same trip). You just have to pack a little of everything.
Lines are pretty nonexistent. Crowds are low-sometimes spooky low(I find it kind of creepy to be walking in the parks and not see other people for a while). You will be able to do alot of rides....which is good because the parks will close early. 6 p.m. sometimes 7 pm close time. You can use your nights for dining at all the restaurants, shopping or just resting up for the next day.
I love to visit in January because it gives me a break from winter and I don't have to deal with crowds or lines.