sorted by prefference:

1st - HARDWARE VIDEO AND AUDIO CODECS SUPPORT!!! This device CAN run DVDs even with full sized XviD/DivXs... And it MUST.
2nd - VIDEO INPUT AND OUTPUT!!!!!
3d - LARGE battery - at least twice larger but better with 4..5 Ah
4th - working driver for WinNT to easily set up PAN over BT or LAN over USB - without any more clicks just Plug-&-Play
5th - two full size SD card instead of one mini-SD - but with same size of the entire device as N810 has
6th - rotation weel at left to easily scroll texts
7th - give us radio back!
8th - joystick near the screen, but not on keyboard - then keyboard can be larger and more useful
9th - more durable case (look at iAudio COWON A2, A3 - if you will drop it it will just jump, but if you will drop N810 it will be the end...)
10th - second camera at back - to allow video shots and Net sharing
11th - 3D acceleration and Compiz/Beryl-like interface using it