I like it a little warmer than you do and my favorite months for times that I have visited are March and April. I would love to try November too - haven't been yet then, but average temperatures, lower humidity, and lower crowds make that look like that might be a great time to go down there, and it's a time that Josh from Easy WDW really likes. I think he really does a good job of stating pros and cons of various times on his site. Weather would be the biggest factor for me if I had total flexibility. On paper to me, November baring Thanksgiving week looks like it might be a really great time (no personal experience with that month though).
Costs aren't as big a deal to me, as I like to stay offsite and with Orlando so overbuilt and the fact that I often do owner rentals too, for accommodations there are great deals any time of year. Orlando offsite is a bargain any time of year compared to other places that I travel. For me personally (just my opinion, of course) and 40% of all guests make other choices -- onsite is overrated and overpriced and that 24/7 Disney immersion and magic to me is a brilliant marketing myth. I can see, though, that if you are going to stay onsite, lower costs of accommodations could be a really big factor and a difference for a trip being in or out of budget.
Probably just because I've been to the parks so many times there is not all that much that is on my to do list in any one park or overall in all of WDW, so crowds aren't all that big a deal to me. Plus I'm an early riser and rope drop person. And even with my baby lists, I use
www.touringplans.com to keep lines minimal for the few things that I do, and do them in a good order. That means I'm really OK as long as I avoid the super heavy times. I have no problem whatsoever with crowd levels of 7. Actually the main thing I prefer to avoid if I can are the 9s, but especially the 10s (I'm getting too old for those -- they are insane).
The Jan to Feb. period you like is a little cooler than I like.
I love March and early April -- have probably though the years done more than a dozen trips in that timeframe and find the weather to be glorious.
I've had really good luck before in the Mid April to Memorial Day period you mention. Averages though are getting a little hotter than I like.
I don't like summer for the heat and humidity. Summer in central Florida has no appeal to me whatsoever. Sept. and October while better than summer are more hot and humid than I like too. Lots of people really like these times, but the humidity IMHO just really takes a lot of the fun out.
As stated before I have no personal experience with November, but on paper it looks like a fabulous time.
December -- I've done a bunch of trips this time of year. Between Christmas and New Year is really insane. Weather wise that month has been all over the place for me. I've had some great weeks with mid 70s, low 80s even, and low humidity. But I've also had other trips where it was down in the 30s, and even though I had my parka with me, it was still just soo cold in the parks at night. It's too iffy to be a first choice, and averages are cooler than I prefer.
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Bottom line though: When is a good time to go to WDW? Whenever you can go and others you'd like to travel with can go. lol.