The case should replace the aluminium cover, and in the same way - ie there should be a cutout for the piece with the connectors and the case should be made padded so it was level with the case on the sides of the connector "block". Possibly there should be a flap of some kind over the connector block - possibly that flap could be part of the screen cover if that was extended to wrap around the device and snap closed on the back of the device. That would protect the connectors and the screen when the flap was closed and expose both of them when the flap was open.

There definitely should be a flip-open cover with some form of protector for the screen, ie if the case is leather it should have a built-in shield over the screen to protect it from impact. Opening the case and getting at the machine inside should be quick and easy as that is needed many times a day for most of us.

There should not be a magnetic closure on the case - especially so if there is room in the case for credit-sized cards. Plain old snap closure would work great.

If anything is embossed on the case it should be something cool and professional. "Nokia 770" - fine. "Don't panic!" - not so fine. :P I don't mind people personalizing their cases but not many would be comfortable with "Don't panic!" on the case, even if they do like Douglas Adams.

It might be helpful if the case had some sort of way to hook it to a belt. Krusell Multidapt is a great system for instance - you can hang the case from a belt in a way that allows it to swivel freely without being in danger of falling off the belt, there are other similar systems out there.