we were down in early december when our son was 12 weeks old. The baby centers are awesome; I thought the one in Epcot was the best, and we didn't use the one in AK, but they are great. My son is breastfed and gets formula, and it was nice to be able to nurse him during the day so I didn't get too uncomfortable. I did make the mistake of not bringing my pump, which I will be bringing when we are down next month.
we also used our big travel system in the parks; it was the first time that our son was in the stroller without the car seat, and he really seemed to love the freedom of being able to kick his legs out etc.! We also tried him in the car seat strapped into the stroller, but we were sure to take him out of it often, which was a bit of a pain.
The main thing that I hadn't anticipated was that he was too young for sunblock -I had asked the pediatrician after I bought the baby sunblock- so be sure to keep your little one covered up. I also bought a couple of cute bucket hats for Joel at Babies r Us before we went, and he used light blankets- it was unusually warm for December- in the 80s and sunny every day we were there. I also packed too many shorts for him- I should have packed light pants so he wouldn't have had to have the blanket all the time. we're hoping it's a little cooler next month...
don't be afraid to take you LO on some rides too- Joel LOVED POTC, and the train, and journey into your imagination- he loved the lights and the music everywhere and we can't wait to get back too!
Have fun!!