As PPs have already stated, the weather is unpredictable for those months. One day, it could be in the 70's & the next day it will be in the 50's for the high (keep in mind to always add the humidity factor, which is what makes the cold in FL feel that much colder). I grew up in PA so I had been used to pretty cold winters. When I moved to FL, I didn't bring a lot of my winter clothes/coats. I quickly learned that because of the heavy humidity, the cold feels that much colder as if it could penetrate your bones at times. I think some people don't take it into consideration for the winter months but think how it is in the summer w/that sticky, wet heat just opposite. Sorry about the weather rant..
Anyway, February tends to become a bit warmer so if you're concerned about that, go mid-late February instead of January.
Crowd-wise, since New Fantasyland has been completed, it seems the 'slow season' doesn't exist like it used to. It will be less crowded but don't expect it to be near empty as it had been in the past for this time, especially at MK.