The ambient light can be useful. You can toss the N810 in a corner, on its battery, for example in your sleeping room. When you wake up you'll see based on the light whether you have an e-mail.

I use this (in the weekend mornings) for a specific e-mail account which I use only for important communication. If the light doesn't blink, I know I don't have to check my e-mail in the morning. YMMV.

Ah, not the LED (which the N800 has too, behind the scroll-pad iirc), but there's a light sensor which is supposed to do something - alter the backlight level depending on ambient light levels? I've not noticed it doing anything useful mind you, but it's a good idea.

The reason people dont notice the light sensor working is because it is doing its job well! If you are viewing the screen in low light conditions it looks as good as it always did, but the actual backlight power is much lower.

You can see it working for yourself. Just you look at the screen in a brightly lit room, then put your thumb over the light sensor. After about 5 seconds you'll see it go much dimmer because it thinks you are in total darkness.

To add to this a bit:

I find I'm always putting my fingers over the light sensor and dimming the display. Poorly placed.

What is the point of the forward-facing camera? I think the pop-out and swivel deal on the N800 made more sense.

I wish these things could flip between Portrait and Landscape modes. I find holding it horizontally for any length of time very tiring.

i just uninstalled the lcars theme it was cool at first but got annoying with the sound effects plus im not a star trek fan

You can. Search for/install sliderotate.

The size for one. The N810 fits into a normal size shirt pocket (might even fit in a pencil protector... haven't seen those in a while).
The keyboard is nice, though a bit awkward, should have a 1 or 2 mm of space at the top. The OS interface for typing still way too kludgy (e.g., predictive typing should be selectable on/off).
The GPS is not that fast, but for me it was quite adequate. Having it in a single unit does make a difference. Having said that, I mostly used it in the car, only a few times while walking (used it for some ag research, and it was quite useful in that setting).
Display worked adequately in sunlight (transflective screens are compromises between reflective and transmissive).
The camera and light sensor are under utilize. Few if any applications use the camera properly, and most any cell phone camera is superior in resolution, led flash, etc. The light sensor... not sure what it is there for, but I used it to read nits and make shade cover calculations (small python cludge). It is not used to turn on/off the led backlight for the keyboard (not sure how that works, as it comes on and off in a rather erratic way), nor is it used to adjust the screen's brightness (another nice idea badly implemented).
Skype works well, so does Fring, but with the recent official support Fring on the iTouch... and the badly implemented BT HID interface on the N810, I have no idea why I would get one again (WIMAX or no WIMAX).

I don't have the N810 anymore, but still have a N800... about the only thing I have it for is to listening to news and as a second Skype machine I keep in the kitchen... thinking about putting a strong magnet on the back and putting it on the fridge's door (as it has become an internet device for my kitchen).