My complaints with the n800 are - wifi only, too big, too slow, no real keyboard, not refined enough, sucky camera and immature apps. The n900 has cell ability blurring the lines between a smart phone and a MID, it's the size of an iphone which also means the screen is 3.5" instead of 4.15" but I'm OK with that I guess, seems much more polished and has the same 5MP camera as the n97 phone. It actually seems like the n900 and the n97 occupy the same space. It's my prediction that nokia used Maemo as a test to see if it's worth the time and have now decided it's the way to go. Look for all Maemo phones 5 years from now. You heard it here first.
Advantages over the n810 - the CPU is twice as fast overall and is the same as used in the iphone 3GS and the Pre. They OS supports the hardware accelerated 3D (finally!), the camera is great, it has a keyboard, it has video out (thank you nokia) and it can charge via USB! Such a simple thing and yet the n800 series couldn't do it. Overal it's nearly identical size to the iphone and it only lost a little over half an inch in screen size.
Advantages over iphone - same cpu as the 3GS, much better camera, it's Linux so not closed, it has a keyboard, the battery can be swapped (killer!), and you can expand the memory. Oh and you won't have the Apple overlords telling you that you can't use Google voice on it.
Here's a review of the prototype at Mobile Review.