It would be nice if the N900 had a slightly separated keys with defined edges and a small 'bump' or different texture on the 'f' and 'j' keys so that they could be found without without looking. I'm thinking a fifth row of number keys will put the device too off-balance when using its stand.
I would rather the keys be shrunk and a small numeric keypad be placed to the right of the alphabet keyboard! This would probably require that the Dpad/menu-key be moved.