Talking about artwork, it should contain:
- the (default) pins used e.g. ethernetshield should mark pin 10-13 a bit like on - http://shieldlist.org
- a URL
- a version number
also the outlines of any devices that would need to be soldered onto the board. If the device has an odd shape, make sure that the shape is defined in the art/screen, to reduce any confusion as to how it is mounted. For Diodes, depict where the stripe would be oriented. For LEDs, have the "flat side" marked, or have circle/square pads to show anode/cathode placement.
I currently have a MakerShield, but I am slightly unhappy with the placement of the additional power and ground headers. They are on the same side where the barrel jack and USB port are on my Arduino Uno below it. The LED/Pot ports are on the opposite side. I wish those were swapped so I could use shorter jumper wires to plug into my adjacent breadboard(s) and not have to worry about the USB or power cable getting in the way.
Another suggestion would be to make alternate versions of boards that are currently available, like my MakerShield alternate header placement example above. See what people dislike about current Shields and consider making a version adjusted to their needs. Also have a decent website with plenty of information on the boards.