As Brian Tilley here above, I've had both the D300 (bought second-hand) and the D700 (bought new in the beginning of 2011) alongside for a while, about a year.
I quickly realized that most of my shooting "slid" from the D300 to the D700, effortlessly: I had a vast array of lenses, from AIs to AF-S VR, and I shoot mostly landscape and architecture, so the 1.5 crop factor was not really important to me. I finally sold my entire DX collection this year in March: a 18-70, a 18-200VR, a Sigma 18-50/2.8 and a 35/1.8 DX along with the D300 itself. The money I got from that deal offered me a 28-300VR and a Fuji X10 - I had to add about 100 Swiss francs to make the deal.
As has been stated, the main advantage is high-ISO and low-light, but I also found it easier to shoot wide-to-moderately-wide landscapes (a Sigma 12-24 and a Nikon 17-35 are among my favourite lenses), and the depth of field is thinner, making for nice portraits (50/1.8, 28-70/2.8, 85/1.8, 80-200/2.8). The only domains that I am now "less well equipped" to tackle are wildlife, but I don't do much of it, and macro, and I really do it seriously only in the spring. My dedicated lenses for those two fields are an aging 80-400VR and a Tamron 90/2.8. I am thinking of getting rid of both to fund a longer macro lens such as the Sigma 150 or Tamron 180, but I am in no hurry for that.
All in all, I'd encourage going FX for high-iso (but a D7000 DX camera has good low-light, high-ISO capabilities), wide-angle shots (but there is the Nikon DX 10-24 and an announced Samyang 10mm!) and thinner depth-of-field, and of the three reasons here only the last one is substantially impossible to reproduce in DX. All the rest, as I have just shown, is not limited to FX, so there is really no magic: sensor technology and embarked processing power are progressing by huge steps, as is proved by the difference between a not-quite-ten-year-old Coolpix 5400 and a brand-new Fuji X10: I would never have guessed that I could use a compact camera at ISO 1600, or even 3200, with results that actually beat those I got with the D70 of 2003 and equal the 2005 D200!
Another, more decisive factor would be an existing set of lenses; such was my case as I inherited my father's Nikon arsenal when he died 10 years ago, and it was a pity not to use that 28-70/2.8: a 42-105 is not a range I favor
Olivier Rychner __________________________________________ Jetez un oeil à ma galerie if you feel like it! And it's a bit void as of now, but I also have a Nikonians blog
Auta i lomë! And my Nikon's only awaiting daylight...