Firstly a big congratulations, wonderful news!
I have taken all mine at varying ages, the youngest being 5 months. while we were there no problems really, we were able to carry him on to so many rides. the time of year is great as there will be no extreme heat for bubs to cope with and no crazy lines, although fastpass and babyswap help no end.
my only problems i remember is where pushchairs can not come into lines or shows, and there were a few wriggly incidents at shows, he did not want to sit still with us when he was tired, wanted rocking a lot so that was tiring. we played pass the baby so we both got to see. the restaurants were more flexible as in if baby was clearly asleep then they let us bring the pushchair in but otherwise they were quite firm about it.
the flight again was a bit of pass the baby but we got a skycot from virgin and were able to get him to sleep a lot of the flight. but any turbulance and we had to have him on our laps whether he was alseep or not.
so overall a really positive experience and we have some awesome video of him babbling to daisy duck and his little face looking in wonder at so many things. and characters and cast members made such a fuss. the baby care centres in the parks are wonderful btw and have everything you could possibly need, fantastic staff, but be warned that if you were to forget something supplies come at a disney price
sorry to point out a couple of downers but i wanted to give you the whole picture. but i certainyl dont think these are problem enough to take away the enjoyment and they truly were minor. hope all goes well with the baby and happy planning x