"assets" folder; stores resources such as language packs and sounds.
All music discs now use .ogg format instead of .mus format.
"library" folder; stores Java libraries such as LWJGL and jinput.jar.
"versions" folder; there are file "versions.json" (meta data) and folders that stores minecraft.jar for each versions (but its name is same as its version. e.g. for 13w16a, "versions/13w16a/13w16a.jar").