I think a case that allows the option of having the device "on" but with the screen protected would be nice, but I'm not sure how you would implement it.

Also, make sure that we don't need to take the device out of the case to connect USB/Headphones/Charger, possibly using a flap of some sort.
Oh, and the microphone should be exposed, so that sound is not muffled. A small hole in the case could acomplish that.

As for MMC, I never swap MMC cards, so I am not fussed about that. An ID window would be good, but I don't want a case that doubles as a wallet.

Mounting for the stylus is a must, as with the 770's stylus holder on the rear, I would prefer not to have a "hole" in the rear of the case. Nokia provided 2 styluses (stylii?) anyway.