At the bare minimum, all you need is a mission file (.mis), a mission lighting file (.ml) and a description text file and preview image. You can point the game to terrain files, skies, textures, etc in other maps for the rest of the content. If it's not default content though (bedroom, kitchen or slopes textures) make sure you include it, since people may not have the other maps.
This is a good way to save space in night and day versions of the same map.
Here is a list of the stuff you'll need for a typical map and will probably cover 90% of the maps you ever make:
terrain file
mission file
mission lighting file
sky resource .dml
preview image
sky, water and ground textures
description file
Basically, you need to include everything that you make custom.
Here's a list of absolutely everything you need to include with a map, for reference. Obviously you only need to include the bits you're actually using.
-key map files
-mission file
-mission lighting
-terrain file
-sky resource.dml
-description text file
-preview image
-custom textures
-terrain textures
-water textures
-sky textures
-interior textures
-static shape textures
-foliage textures
-custom particle textures
-precipitation textures
-custom textures for sun and other objects the move through the sky
-anything needed for a custom addon that loads with the map
-bits of code
-triggers (you should include these as an addon so that other people can use them in their maps, although you can embed the in the map mission file if you really want to)
-precipitation (autumn pond, for example, had rain)
-custom addons (autumn pond, for example, loaded a ridable snail)
-random other files
-sounds
-blockland save that loads with mission
-anything required by custom addons
