So after a year and a half, N900 needs to:
1) Have an almost perfect keyboard you can use as fast as a computers one (n810 keyboard is not sensible enough).
2) Cool interface.
3) Great connection with your Nokia phone: That means you shouldn't even take your phone out of your pocket to use it: PIM editing and call through bluetooth.
4) portrait and landscape modes. Shouldn't have to use both hands.
5) Cool exterior. Must fell durable.
6) Intelligent elections: Use of MiniUSB instead of MicroUSB makes it unnecessary to carry arround a wire. Use of SD Cards or MicroSD cards make you able to have more HD available or at least it doesn't make you carry an stupid Mini-Micro converter.
7) A little bit bigger screen will not harm if weight is not affected.
8) Look and Feel: All major projects such as Canola or a PIM should feel integrated.
9) Abandon stupid restrictions: Surely Phones/iPhones way to startup aplications are better for a touchable tablet that the windows/kde like Start button of Maemo (needless to say that the three start buttons are horrible). Don't be afraid to try new things but never be afraid to turn back when those prove to be worse.