Although 12mp sounds a guarantee to great images, that is only a part of what the camera is about. In truth, for most purposes you will never see much difference between 6mp and 12mp, though it does allow you to crop substantially and still have an image capable of a good print or publication at a sensible size (which is mighty handy in the bird photography world)
The Nikon D2x, unlike the big Canons, still uses the same sensor size as previous Nikons, so it does give this 12.8mp + the 1.5x crop factor that occurs when you attach a 35mm based lens.... great for telephoto photography, but not good news for landscape guys.
I have never owned a pro body camera before (even my Nikon F100 was not that calibre), the obvious difference is that they weigh a lot and are built to stand up to the rigours of hard daily use... as well as being sealed well enough to withstand the elements.
AF speed is suitably faster than with most cameras, as well as almost non-existant shutter-lag and the camera ready to take a photo almost the instant you switch it on. Responsiveness is the keyword here, it is desirable in the world of amateur photography... but absolutely essential in most areas of professional photography. Frame rates of 8fps may seem over the top but the duration of a spectacular scene is often short, so it is vital to cover your bases with as many shots as possible... it could be the difference between a £50 shot and something that will help secure your financial security for life.
One thing that is amazing is the battery the d2x comes with, at 67% after two days of shooting and playing around (yep, you can check battery life to the % point)... god knows why i purchased a spare.
One aspect of having so many densely packed pixels in the sensor is one of movement blur. A tiny bit of camera shake will cover more pixels than that on a less densely packed sensor, stability is vital.