Wishlist
A 6x4 inch screen with landscape/portrait. Maybe a little bigger to hold an "action bar" for fat fingers. OLPC screen (black and white readable daylight screen). 10 hours standard battery life. one or two USB slots (with caps). Wireless and bluetooth. The battery life can go down with wireless/bt on, but it must be possible to turn them off completely and get 10+ hours.
Virtual keyboard like the n800/770. Especially like the "fat fingers" version.
Lastly (and to a large extend, most importantly), it should be rugged enough to take the knocks a book can manage and it should be weatherproof.
Ruggedness. If your hands are wet or dirty, you're not killing it when using it. You can leave it on the grass even if it is wet grass. If you accidentally kick it while there, it won't crack. If you hoof it deliberate, well, it's not a football.
Screen. Enough to be a real book and be used as a book reader. A paperback sized one may be a little unwieldy to have in a pocket, but 6x4 is a pamphlet and still acceptable. And being visible in daylight means you can USE IT as a book.
Wireless/BT because I don't think anything would manage today without them.
USB. If anything is missing, USB can supply it.
Virtual keyboard. A hinge is a weakness. It makes it thicker to little real need. If you need a full size one, USB or BT will cover it.
Probably want a proper clamshell to store it in when not in use so that you can stuff it in a backpack and not care about your keys/penknife/running shoes breaking anything. Make it strong enough to handle going through luggage checks or having other bags laden on top of it in the clamshell and you've got an investment that won't disappear because you were careless.
With that, there's not a lot of need for more than 10GB spare storage. Memory dependent on OS, but a Gig would be enough.
The ruggedness is most useful for me. These devices are for wherever they need to get used and getting them there is not going to be sinecure. So the EEE PC being built pretty much like a laptop (with the concomitant holes in it to let flavour flood in...) doesn't rate high enough to BUY it. Screen is bigger than it needs to be, so this is more an A4 book-a-like and you now KNOW you're carrying it. Battery life is close and could come closer without the size of screen they have.
Pick and mix gets you close except they ALL seem to lack the ruggedness. If you had any of them, you'd be worried they were going to get damaged if you put them in your bag. And who needs another worry?