The three major factors that, for me, make this camera (2 of them apply to the D300) so incredible are . . . well, the full frame sensor (finally), the outstanding low-noise/high ISO performance . . . and the extraordinary color! The photographer has a greater deal of control over color than ever before and the results are stunning (use Picture Control). And noise (the digital equivalent of film grain) is at least a 2-stop improvement over the previous D2x. Actually, it's probably 3 stops, but say 2 to be conservative. Believe it or not, ISO 6400 is actually usable! (The D300 is one stop less)
While those features were the most compelling ones in my view, there are plenty of others that are a big plus, such as:
~ 9 frames per second ~ 51 point AF ~ Live View ~ 12mp CMOS sensor, self-cleaning on D300 ~ A new 3" LCD--about 4x the resolution as previous LCDs.
The list goes on, but you should read the tech specs and professional reviews. Suffice it to say it is loaded with features and handles like an absolute dream!
Compared to Canon's $8000 professional models, this camera has a very clear advantage. Sure you lose some megapixels compared to the MarkIII, but that's not so important for the majority of (but not all) photographers.
Another thought . . . while this is a good deal at $4999 compared to Canon, you can get almost the same camera for $1799, with the D300. If you don't need a full frame . . . or the extra 1-stop high ISO, the D300 offers about 90 - 95% of the features and performance of the D3 for 40% of the price! If you want to get the best possible picture quality for the lowest possible price, consider the D300 and put the difference ($3200) on some of Nikon's great new professional lenses.
As of this January 2008 writing, this is listed on Amazon at $6399 through Cameta Camera. PLEASE DON'T buy at this price and allow yourself to be exploited by the backorder situation. Instead, pre-paying $4999 for it at your local camera store puts you ahead of everyone else on the waitlist who merely put down deposits on it. Or wait until it comes down here on Amazon. Again, $4999! That's the price. There is no need to pay an extra $1400 "sucker fee" for it just to have it a few weeks or maybe a month or two earlier. You survived until now without the D3, so you can make it a little longer--just don't pay more than retail. $4999! $4999. $4999. Get the point? :-)
Post Script, April 16, 2008:
I notice this is now being sold by 17th Street Photo, who I've not done business with, but at least they are charging the *correct* price of $4999. First it was a $1400 markup, then $500, and it's good to see the price gouging attempts appear to be dying down.
Post Script #2, November 2, 2008:
How times change--the gouging is gone, and so are the backorders. The price has dropped below $4500 I see. My opinion on this camera remains unchanged in the 11 months since I first got my hands on it, and it's as excellent as it ever was, but now, with the introduction of the D700, I would have a hard time spending the approximately $4300 it now sells for, when the D700 is selling for $2699. The D3 is better built, and shoots 9 fps vs the D700's 8 fps with battery pack, but I don't see it being worth the added $1600 cost unless you're a professional who shoots every day.