A lot of the negative comments I see in this discussion are about the slow to respond UI, battery life, and the lack of a "retina display" level of pixel density.

The slow UI problem was licked some time ago by the Thumb2 project (porting a huge portion of the OS & apps to the more compact Thumb2 architecture), which freed up an amazing amount of RAM. The slow UI problem, you see, was cause by swapping (to flash). I switched to the Thumb2 CSSU some time ago and it's nothing short of astonishing. The difference is much bigger than when I overclocked my 600MHz N900 to 1050MHz (that didn't do much for the UI, but it did eat the battery faster and make it run hot). As a nice side benefit, the lack of flash-burning (swapping) resulted in ~3x battery life.

The pixel density of the device is actually too high for me (my eyes are too old). My N800 (and 770) display had the same resolution at 4.3" instead of 3.5", and I can still read small fonts on it fine. The N900, on the other hand, is barely usable as a computer anymore (remember, Debian based OS - real xterms). I'd like to someone over 40 try and read an xterm on a 5" 1080p display without bifocals.

The Neo900 project handily solves the 1st problem in hardware... one of the biggest problems this device had wasn't the CPU, it was the 256MB of RAM (it needed ~320MB to run efficiently - my rough guess based on the difference the Thumb project made).

Unfortunately, the screen density issue is (obviously) not addressed by a motherboard replacement... and the price is sadly too high for me to afford.

As to the "why?" questions about the device:

Most open phone OS I've seen on any commercial phone (I'm not talking about a limited hacker-marketed Neo1973 here). I can get root on the device without breaking any laws or EULA anywhere... as it's not restricted from the owner. I can modify how the device works easily (many functions are event-triggered via D-Bus, which opens amazing possibilities). A minor, but very useful example is the remapping of the camera button (when the lens cover is closed) to bring up the application menu for quicker multitasking). It goes on and on.

The most open phone device I've seen (same disclaimer as above) with the ability to run numerous OSes, multi-booting between them: Fremantle (stock OS), Mer, MeeGo, Tizen, Debian, Ubuntu, Android, possibly more (yes, I know #2, #3, & #4 are related, just as Debian and Ubuntu are).

FM Receiver (yeah, I know)

FM Transmitter... turn any car radio instantly into a speakerphone, or play any music on your built-in 32GB of storage (or 64GB of microsdxc) through your stereo, amp up your games, etc.

TV-out... play games on your TV.

IR transmitter... a really versatile universal remote. ;^)

Did I mention 32GB of built in storage and a microSD slot that can use sdxc cards as well as sdhc? Remember, this is a device released in 2009!

USB-OTG... use wired ethernet(!), optical drives, joysticks, USB-sticks, etc. (anything over ~100ma requires a powered hub as the device won't supply more)

Resistive touchscreen with a stylus. Yes, I consider this a plus as I find it absurd to try and click on a small cluster of pixels with a finger that covers tens of thousands of them. My finger is fine for dialing, but outside of that, I want accuracy. I'd much rather type with a stylus on my old N800 (pre-N900) than with my fingers on my HP TouchPad (~10" capacitive touchscreen).

Camera with Carl Zeiss Tessar optics and a sliding cover. While the stock software could only record 480p video, a modified library can be installed that allows 720p video recording (uses a mild DSP overclock, IIRC). The fact that this sort of modification is possible without the assistance of the manufacturer is a testament to this device's versatility.

Video calling (more on that below)

Deep integration of communications via plugins (highly expandable). Want to IM someone? No need to dig for the right app for their choice of IM providers, just IM them - all protocols are handled by the same UI (even IRC!). Take a picture, share it via whatever sharing service you wish (using the same camera UI), from Facebook to Twitter to many, many others (I never used this feature, but always thought the way it was integrated was impressive and elegant). Want to call someone? It doesn't matter if you're calling using VoIP, Yahoo-IM-voice, POTS, Google Talk, Skype, etc... you used the same phone UI to call them. Deep integration, done extremely well.

OK keyboard (not nearly as nice as the ol' SideKick II, but I've found nothing that compares to that keyboard).

For its day, it was pretty powerful. The closest device Apple produced was the iPhone 3GS (same CPU, very close to the same 3D GPU).

http://en.wikipedia.org/wiki/Nokia_N900

Edit: Clarification of calling integration.