I can certainly tell you it wasn't a luxury, I had to set aside cash for an entire year and sold my Canon kit to cover 2/3 of the price. Otherwise I wouldn't be able to afford. And I didn't buy it because it was a pricey camera or because I want to brag, I bought it because I needed it.
I wanted the video features, I had a 5DII before but trying to invest in lenses and accessories for 2 different systems was more costly, so I wanted a Nikon camera that could get the same video features, the D4 was the first one ever produced by Nikon that had what I wanted. My other option was the D800, but I doubt I could handle the enormous file sizes of a 36MP sensor.
I was initially hoping that the D800 would be more around 16-24MP camera, when I heard it was going to be 36MP, the only option left for me was the D4.
Two things that immediately struck me as being great on the D4.
1) The new autofocus system, very fast and very accurate. Let's hope that this system gets adopted into new consumer level Nikon bodies too.
2) The larger better LCD screen, I was never a fan of using live view before now. Serenity was shot using a 85mm f/1.4 lens with manual focus. Nikon has certainly improved from the D90 I first owned that had live view.