1) Main Themes. I hear two big ones. The happy adventure theme (Overworld) and the evil danger theme (Underworld?). The happy theme is heard in Overworld, Underwater, Underground and Athletic levels. The evil theme is in the Ghost House and Castle. All contain either of the main melodies, just with different meters and textures. Crazy!!
2) They are heavily melodic. And that's why we hum them!
3) Division of Themes. Anyone notice that the level types are divided almost exactly like the very first Mario game? Overworld, Underground, Underwater, Castle. The only exception to this is the addition of the Ghost House and Athletic levels in SMW, and even those borrow from the Castle and Overworld themes respectively. I honestly didn't completely realize this until I started this project. Double crazy!!
4) The map themes are NOT heavily melodic. With the exception of the Forest of Illusion and Donut Plains, most of the map themes rely on simple harmonies + rhythmic interest. For example, Yoshi Island theme, which to me sounds like a basic call-and-response between the treble and bass lines. The lack of melody in the map themes makes sense though, you probably shouldn't have the map music overshadow the music of the actual levels.
5) I am having way too much fun :-D
Enjoy the new Super Mario World arrangements for the Main Titles, Yoshi's Island, Donut Plains, and the Overworld! A LOT more are on the way!