Your comment about DOF possibly being an issue is important. Moving from DX to FX, you are losing about 1 1/2 stops of DOF. That's good if you want a soft background, but bad if your depth of field goes from 8 inches to 4 inches.
The D800 will reveal any softness in your choice of camera settings. The D7000 had a bit of this issue as well - thus the early complaints about softness. A 10-12 megapixel camera may not outresolve your lens, but a 36 megapixel camera is much more likely to show a clear difference based on aperture selection, lens quality, and use of teleconverters.
Don't get into the trap of automatically viewing images at 100%. It's okay, but when you look at just 1000 pixels of a 7200 pixel width, and blow it up to a 15 inch wide display - you are looking at the equivalent of a 7 foot wide image from just 18 inches. Viewing at 100% is okay - but slightly soft images can look just fine sized for normal use.