chngpe01 gave a very good round up, summarising many important aspects that most people will overlook, or at least, seem minor.
In my other post
here, I've spoken about ISO comparison with the current crop of cameras, and why ISO is not the only point to look at when focusing on a DX vs FX debate. I've also wrote about international repair, but that I think is not of concern to many people.
In these 2 posts
here and
here, I've spoken about inter-camera compatibility, within the DX format. Again, if you are using only 1 camera, this may not affect you. But take note that if you go on an upgrade path, and have spare batteries it may affect you unless NIkon changes everything to the new battery.
Lumiere speaks about image, professional image (not images coming out of the camera!)
here.
Basically I would caution against going into a "buy buy buy, upgrade upgrade upgrade" frenzy and dissing all other options.
High ISO is starting to become the hype that megapixels used to be a few years ago.
Carefully consider what you want, and perhaps need and then re-evaluate your purchase options.
One thing I'd like to add to chngpe01's post about focusing speed - the single digit D series cameras have a faster motor in them, if you are using traditional AF lenses (lenses without SWM).
No matter how fast your focus on a double digit or triple digit camera, you will notice an difference on a single digit professional camera if you use AF-D lenses.
Even on a D700 and D3 which have similar processing power, I can detect a performance increase on a AF-D 80-200mm f/2.8. (D300 has CAM3500 also, but slower processor).