When tiles are square, they can be rotated by 90, 180 or 270. This would avoid having to create the same tile 4 times in the tileset. This is particularly usefull when tiles are roads/paths etc.
I know it is possible to rotate in the Objects layer, but the above feature request is for the map layers.
I understand this would require to store some property for each tile on the map, but maybe it could be stored as a rotation array for all the tiles in the Layer Properties?
Right, as @Ablu points out this feature is already there. The X and Y keys flip the current stamp on the respective axis, while Z and Shift+Z keys rotate it by 90 degrees in different directions. Flags are used for each cell to store whether the tile graphic should be flipped horizontally, vertically or diagonally.