In no particular order, my top 12:

1) native calendar (can be a cheapie one, people can get Dates or GPE Calendar for the full fledged one), including todo list

2) sync data in some fashion that is compatible with Thunderbird, Sunbird, and Firefox (bookmarks, contacts, events, todo's)

3) 2nd USB port, in host mode, charge via USB on the client port

4) universal "spell check as you type" from a built-in dictionary, that works across all of the native apps (notes, mail, browser, google-talk), and is available as in the API for add-on apps.

5) video out (pref. mini-DVI with DVI-A support so you can use mini-DVI to VGA and mini-DVI to video cables.)

6) support for bluetooth stereo headsets (with and without microphones)

7) slide'n'tilt keyboard (that can be tilted without sliding, to replace the built in stand), retaining the current buttons on the face of the device (not moving them to the keyboard)

8) Bluetooth PAN, and whatever support is necessary to transparently support storage from bluetooth hard drives (Seagate DAVE, Agere BlueOnyx).

9) software and bluetooth support necessary to use the N810 as a handset/controller for a cell phone (display caller-id, dial calls directly, dial calls from contacts list, answer calls, route voice in/out through N810).

10) finish the mini-mozilla browser, and add a mini-mozilla mail client (or, at least an IMAP client that delivers the functionality I keep talking about).

11) transparent to the app (except that it gets told to re-size itself to fit the new aspect ratio) ability to use both portrait and landscape. Doesn't have to detect via accelerometer, it can be button induced.

12) small user facing camera for chatting, camera/video (cell phone quality) camera facing away from the user (on the back of the device).