Everybody seems to be asking me this question. People ask me is Nikon the professionals' choice, is it the better brand, does it produce superior images? I simply reply, Nikon is my choice because I like the lenses available, the very accurate metering system and focus tracking, and to me Nikon is very ergonomic plus I really like the colors from my Nikon DSLRs. Other DSLR manufacturers also produce very good cameras, maybe even better (to some people) but to me the combination that Nikon provides is just perfect.
Lenses
While generally more expensive than the competition, Nikon lenses even the consumer models produce very good results. Stepped down to their optimal apertures, even consumer-prosumer models are almost as good as the professional models. With the launch of the new 14-24mm f/2.8 and 24-70mm f/2.8, together with the 70-200mm f/2.8 VR Nikon has seamlessly created a f/2.8 professional lens lineup that stretches from 14mm-200mm. This while also having an unmatched prosumer model in the 18-200mm f/3.5-5.6 VR. Add to this the ability to use virtually all F mount lenses ever produced, albeit in MF or without metering (on some bodies), Nikon produces the most diverse and quality lenses for their DSLR lineup. Nikon lenses just produce great sharpness and color.
Metering
This is very important to me, getting a properly exposed photo is paramount. In covering events, most of the time you wouldn't have the time to set proper exposure or exposure compensation or flash compensation being on the run. You have to be able to trust your equipment's metering. It has to be accurate and reliable, and Nikon's metering system is just that. This has always been Nikon's greatest asset starting with its (then) amazing center weighted metering system started with the early F series cameras. Up until 2005, I still used my Nikon F3HP with great pleasure and success because of its superb metering system. This in fact has elevated the Nikon metering system to near legendary status to the point that users of other systems only wish they had.
Focus and Tracking
Another feature I really enjoy with my Nikons are the accuracy and speed of focus and tracking. In photography as with most other applications, speed is nothing without accuracy, and although it has been said that Nikon isn't the quickest AF system around, I certainly could make a case for its accuracy. I love the fact that the D3 AF can be calibrated to any lens, making accuracy much greater. Using the Group Dynamic AF is also accurate and as seamless as can be, making tracking a moving subject very easy indeed.
Ergonimics
Well, this is a very subjective issue. I just find the layout of the buttons very logical and it is very easy to move between cameras (ex. D200 to a D80, D3 to a D300, etc.) for those who like to shoot with more than 1 body. For the prosumer-pro bodies at least, most everything has a dedicated button and a programmable func. button which I find very useful in the field.
A Few Notes on Noise
For a long time now, high ISO noise (starting at ISO 800) has been the knock on Nikon DSLRs. To be honest, it really didn't bother me that much, coming from film scans, even a fine grained ISO 100 is just about as 'noisy' as ISO 800 in Nikon DSLRs. There is also a quality in Nikon's 'Digital" Noise that was very endearing to me, maybe because it looked a lot like film. But with the release of the D300 and D3 Nikon seems as if they have really closed the gap in this department.
The Colors of Nikon
This is perhaps one of the best reasons to buy into the Nikon system at the moment. A group of my colleagues did a very impromptu test at one of the many photo conferences (I will not name anybody or brands), two of them had 2 brand new DSLRs one was a top of the line DSLR from another manufacturer with a pro grade zoom lens and the other was the dirt cheap D40 with its kit lens, they printed straight out of the camera jpegs, shooting the exact same subject. When they asked their peers which print they preferred, not knowing which came from what camera 8 out of 10 chose the print made by the lowly D40. When asked, they replied 'the colors look more vivid and perfectly saturated skin tones are perfectly captured compared to the other print'. I know this is hard to substantiate but I do see these colors from my Nikons everyday.