Has anyone been in January before? What should I expect? I've never visited in the middle of winter before.
We were there this past January 2009! Coming from Northern Ontario it was way warmer than at home for us mid winter! However...Orlando just happened to be having a record breaking cold snap the week we were there!
I believe one morning it was about 35 degrees Farenheit!

One day (the 21st I think), it stayed pretty cold ALL DAY! We were at DHS that day and all though all rides (except TSM) were pretty much a walk on most of the day, we were wearing hats and mitts pretty much all day!
It did warm considerably over the next couple of days and over the weekend the temps even got into the high 70s low 80s! Expect to have a variety of weather possibilities! We packed a little of everything! We had shorts, T's, long sleeves, jeans, capri's sweaters and even jackets, hats and mitts! We used it all too! We even went swimming a couple of times!
Crowd wise, it was fine for us! I hadn't been to WDW in nearly 15 yrs (pre FP) so I really wasn't sure what to expect. We really didn't have any complaints! The most we waited for any ride was about 35 mins and that was for TSM on standby @ 9am on a Fantasmic Day! We didn't see any of the big tour groups that everybody else seemed to see at that time either!

Maybe I was just too focused on my own fun and didn't notice!
We did Rope Drop for every park every day and used FP extensively and were able to ride tonnes of rides in a single day! On our first day at MK we rode all of Fantasyland (except for Snow White and Philharmagic) AND pretty much ALL of Fronteirland AND Adventureland BEFORE noon! This was after eating breakfast in the Castle!
Here is alink to my TR from last January! I think it is closed to comments now but I believe you can still read it!
http://www.disboards.com/showthread.php?t=2165342
Hopefully it will give you a bit of idea of what to expect in January!