If I build a giant castle made out of Gold, or flood the world with water, will those changes show up on a Map? If it does, that would greatly enhance the use of maps for me.
Maps show the blocks that would be visible from a bird's-eye view. For example, if you made a large canal, it would be visible on the map as blue. The maps are dynamic, so any changes to the world are shown. However, only the top-most block at that particular X/Z coordinate would show – because of this, nothing underground would be visible (since the map would only show the ground above).