My post is based on an aggregate of knowledge I've gained from watching hours upon hours of special features on more than a few Disney movies. So pretty much it's not that in depth.
Walt's time in Marceline gave him a lot of ideas he ended up infusing into his movies, the parks, and the whole Disney mythos. Walt also developed a talent for drawing while living in Marceline. 
He lived in an idyllic setting in a quintessential American town with a main street, quaint shops, friendly people, etc. The Lady & the Tramp special features said that the town the movie is set in is based off of Marceline. You can see the same sort of setup in Mainstreet in the Magic Kingdom. I think spending time in such a nice place as a child left a permanenet impression on him, and probably made him want to create a place where people could bring their kids and feel that same joy. I also think happy memories from Marceline might be partially responsible for the whole "happily ever after" and "dreams come true" schtick. After all, Warner Brothers (back then) and Dreamworks (now) also make a lot of animated shows and movies for children, but they aren't recognized purely for their wholesomeness like Disney shows are.