The pixel density is a remarkably small element of the cost - but a high element of perceived value. The pixel density of the D800 is the same as the D7000 that preceded it. The pixel density leader these days is the V2. Both of these consumer cameras are priced around $1000 - and they have none of the features of the pro level AF, build, or full image frame rate.
I find the D800 to be a great tool when you don't need the frame rate, buffer, or durability of the D4. The D800 excels when you want maximum resolution - as others have said, it's a specialty camera. For static wildlife it captures wonderful detail in feathers, fur, or scales. For bursts featuring peak action the D4 frame rate is much better. For ultrafast frame rates or quiet operation, the V1/V2 has a place. You choose the right tool for the situation.