Try down-sampling some of your disappointing shots to 16 megapixels and compare them with comparable shots from your D7000. Down-sample some of them to 12 megapixels and compare them with shots from your D300. The comparison should tell you something important about your technique.
I should have added: Try the new 70-200 f/4 for some handheld shots. A while back I put up a post saying that on the D800 that lens probably is better even than its f/2.8 brother, but Brian evidently moved the post. The new VR on the 70-200 f/4 is incredible, and the D800 is the best lens testing hardware around.