We've done MNSSHP on Halloween night and the crowd was much more manageable than I expected. If you are big Halloween fans, I think you would enjoy that time of year in WDW. We usually have gone during the Oct/Nov time periods for either Halloween or Christmas decorations. The parks and hotels are all great that time of year. I can't really speak to Spring, but a lot of folks really enjoy the Flower and Garden festival in Epcot.
We are actually considering a trip to DL for the 60th in November, the week after Thanksgiving. However I am considering doing food and wine in WDW and moving DL to early May. Since you are a DL vet what are your thoughts on that?
I think early May would be a great time for DL. The weather is warm, but nice. You may get a bit of morning fog (May Gray) but that just means your mornings will start out cool. As long as you get there before schools start to let out, lines should be decent. If you are there May 1, you get the added bonus of Dapper Day. It is fun to see people dressed up and makes for great people watching... especially at California Adventure since so much of it is retro-themed. It's a bit like being on a movie set.