In the interest of focusing on the practicalities rather than technical specs:

How I want to use an n900

I should be able to take decent pictures. Nothing crazy-good; 3+MP with a nice lens would be fine. Vid-cap would be a bonus.

I should be able to leave my laptop at home more often. Honestly, the n800's already pretty close on this. Some straightforward horsepower advancement and it'll be a suitable replacement for whenever I don't need Windows.

I should be able to get rid of my PMP. Kagu/Canola are pretty close, but I do want more storage. Particularly with the recent improvements to video and the n810's promise of even more.

It should have more internal storage in general. It'd be nice if I wasn't lugging around an external disk or laptop for 20GB of data. This also feeds into the pics/pmp features. More storage makes those features far better. (no, a deck of SD cards does not count)

I want to do (more/better) handheld gaming. The n810 looks really close to 'good enough', particularly with the slide-out dpad/kb. But it'd help to have support of the new NGage platform, 3d drivers and a bit more horsepower.

I want to get rid of my phone. The n810 is small enough for me. GSM+edge is fine.

Barring that, I'd like to leave my phone in my pocket/backpack. E.g. use the NIT and BT to control the phone. Switching back and forth between devices is obnoxious. Particularly when I'm calling people in regards to data I have in front of me on the n800, or trying to keep contact information in sync.

Problems with the n800 I'd like to see fixed:

UI. It should be far more responsive and finger-friendly.
Charging. It should use the microUSB for charging.