Off the top of my head:

1. The E90 has a 6 row keyboard, so the key layout is almost identical to a real keyboard (almost no need for a symbol/function key). The 6th row, at the top, is a row of application specific keys (ie. the "email" key, the "calendar" key), instead of F1-F13 that you get on real keyboards, but otherwise, it's pretty much the same. So, you don't have to deal with the compromises and annoyances that go into a compressed key arrangement, like on the N810. ((Edit: see below))
2. The hinges on the E90 look like they can bend all the way around, so it should also be able to function as a tablet, and not just as a micro-laptop. So, with the right button layout on each side of the screen, you kind of have an opportunity to get the best of both the N800 and N810 experiences.
3. the outer screen is useful for quick and convenient access (like you said), but it is ALSO useful as a real phone keypad, in that you don't have to dial with a virtual keypad (like with skype on the N800), nor dial using a linear number pad (like on the N810). You can hold it to your face for regular phone talking without your cheek pushing on a touch screen (without the need for special sensors, like the iPhone uses), etc. That's useful whether you're talking about a GSM phone, or just a skype and SIP phone.

EDIT: Here's the E90, so you can see how the hinges look/work, and see the keypad layout. You'll notice that there's only a few function buttons, MOSTLY related to the phone functions themselves (it looks like brightness, volume, etc. ... and then the currency symbols and curly braces). Going with what I said about the control keys in my first post about this, if you were to change the "Chr" buttons to "Ctrl" and visa versa, then that'd be the only keyboard change I think I'd need.

E90 pictures