I've happily shot a D300 for four and a half years. If a D400 appears in the next six months, I'll place an order the next day. Otherwise, I'll have to consider going FX with a D800/D800e.
I'm not interested in the D7100 or D600, and I've too much Nikon gear and experience with the design philosophy to want to learn a new manufacturer's system.
I want the pro controls. I don't want a body that wastes real estate on scene modes that I'll never use.
I also want the pro body's build quality. I dropped the D300 with 17-55 while riding a ferry in Turkey a couple years ago. The impact was bad enough; next, the ferry tilted and the Nikon skidded across the floor. Embarrassed, I picked up my camera and carried on shooting, just like nothing had happened, because nothing did happen. The Nikon was unscathed. While I don't make it a habit of tossing my equipment, I like hiking and travelling with a camera that has the D300's level of sturdiness.
I prefer DX over FX as I'm willing to trade some image quality for lighter weight and smaller size. Recently I tried leaving the zooms at home, just grabbing a couple of primes. Makes for a small-ish travel kit with great performance.