My very first DSLR was the D90 4 years ago after a decade gap in shooting. I also had a well used and loved A1, that traveled the world with me. I still miss the ease of MF with it.
I have been through a few upgrades in that period and have enjoyed the D7000 and D800 but none got used as often as the D90 which is still used, when I can get it away from my GF. Each camera produced images that are really cherished, but most are not regardless of the technical performance of the camera. What causes an image to be compelling, interested and valued has nothing to do with resolution, DR, high ISO performance or any of the normal reasons we cite when justifying spending $3300 on a body, and $10,000 on lenses. We usually do not ask what lens would used on a gallery wall exhibit because instinctively we know it matters not a bit. Neither do we ask what brushes a great master used to create images we value a great deal. Anything more advanced than a D90 is a small incremental change, and as you go higher, the incremental steps get small and more expensive between steps. If you look at your favorite prints you have done, there is not much chance you would have liked them more if done in 600mpx and ISO 2.3 million. It is fun to get new toys, in rare extremes, it might even enable a difficult shot with anything else but there are few rational reasons to keep upgrading other than simply wanting the new toy. Seeking a new body with expectations of better images is a low return proposition. Yes, it might be better but it can be seen only in direct comparison at higher magnifications of the same shot, at the same time with the same lens which is not how we view original works. A better investment if the goal in more keepers is enroll in some of the Nikonian workshops or taking an art appreciation class at a local university, or hang out with better photographers who's work you admire. A good workshop on landscape or macro by an expert really will result in more keepers than spending $10,000 on lenses and body. To date, my most liked portraits were done in equal proportions with the D90, D7000, and D800. One of my best ever, hanging as a large print on a gallery wall was a studio portrait using the D90 and 18-105! coupled with some home designed and build studio strobes and a patient model who liked the experimenting going on. Good lighting trumps good bodies and lenses in most cases and luckily good light is cheap and often only requires being creative with things laying around the garage or workshop as tools. If all else is good, good lenses are great to have. And come with great price tags. Good luck and have fun Stan St Petersburg Russia