Physical size.

The N810 can do just about everything I would want/need, in terms of CPU, RAM, and storage, in a netbook. It's just too small, and not ergonomically suited for it.

If the N810/N900 had:
1) Easy way to support an external full size USB keyboard (full size in terms of the key sizes, it could be one of those "full size keys, but no number pad)
2) Easy way to support an external 7" or 9" LCD or OLED display (and such displays were easily available accessories for the N810/N900)
3) Support for displaying larger resolution on external displays

(or, instead of those, redfly compatibility)

then I probably wouldn't need/care about a netbook.

You can sort of do #1 with some adapter magic, but I seem to recall it takes more than just a USB size adapter. And, if you're ok with portable bluetooth keyboards, you don't have a problem with what the N810 does now ... but I haven't found a portable bluetooth I really liked. But I do love my folding mathias USB keyboard much better than I did the stowaway I also have. I would definitely want to be able to use THAT with an N810/N900.

But the real gating items, IMO, are #2 and #3. Without those, I need a netbook for many situations. I don't really need more CPU, RAM, nor storage than my N810 has (and the above netbook DOES have more storage potential than the N810). Those would be nice, but they aren't necessary.

With all of that in place, I wouldn't have any problem plugging the keyboard and display into my N810, and taking notes on that while I'm in a meeting (anywhere I have a table or desk). Or hooking it up to my KVM switch on my desk. I could easily forgo a netbook in that case.