Re: Full Frame
While you are right in saying that the lens is still only a 200-400 zoom, no one really should care what the focal length is. If you were using a medium frame camera then 200mm would show you a much wider field than a 35mm full frame camera would, likewise a APS-C sized sensor will give you a field of view equivalent to ~1.5X that you'd get with the full 35mm frame. If you like, this works by cropping, but then so does a "full frame" camera. A 200mm lens could produce a wide angle view, if you let it project onto a large enough area.
Many photographers are used to thinking in terms of field of view in terms of the focal length they need to achieve it based on a 35mm camera. Hence the tendency to refer to "effective" focal lengths based on their 35mm equivalence.
In terms of your original questions about why aren't there more full frame DSLRs, well I guess for most customers the APS-C sized format is just easier to live with. It allows for smaller lighter cameras, small lighter lens and greater depth of field. For most people the image you can capture on these smaller sensors is good enough. For most people the advantages that full frame gives, such as lower noise, higher sensitivity, or higher resolutions aren't worth the cost in terms of money and size/weight. Few people need to take action shots in the dark, or have lenses good enough to really justify 30+M pixels, even with image stabilization most people would just get camera shake if they looked closely at shots from a D800.
I'd love to have a D800, I really don't need what a D4 is designed to do. I'd like the improved control of depth of field on prime lens. But it would probably only be a few times a year I wanted to take a picture I couldn't just as well take on my D300. But then I'm an occasionally keen amateur and I can easily understand why a real professional might need all the capabilities that a full frame camera gives.
Perhaps there is a hole in the market for a lower cost full frame camera. I guess this is Nikon's go at filling that hole. From my point of view, I'd love to see someone fill the huge hole between amateur zoom lens (typically F5.6 or 6.3) and the professional F2.8 ones. I guess each to their own.