I upgraded from a D90 to FX, but still hold on to the D90 as it's a great camera. For out doors, wildlife, it is more than capable. I guess by going FX you gain better image quality, but probably only really noticeable when viewed or printed large.
Where I find I have gained is shooting indoors, at higher ISO levels, especially for gigs. Or naturally lit portraits indoors or in dim lighting. I have also noticed an extra degree of sharpness, clarity and overall depth and contrast and quality, when pairing the D800 with speedlights, in a studio type setting. I'm also a cropper, I do try frame right on cam, but sometimes, well, a lot of times, later on I feel an image would be better cropped a certain way. Too much nothing space or whatever reason. Shooting FX allows you much more freedom in that regard too.
SO, better overall quality, better ISO performance, better AF with more points, and tougher body + greater resale value.