Welcome to Nikonians. I can't answer 1 or 2 since I moved straight from a D90 and D300 to the D600. For #4 the answer is a qualified yes and no. I grew up with 35mm and DX was often a squeeze for big picture shots. The FX frame is so much more expansive that I find, for example, that the 85 1.8D, which was hardly useful on DX bodies, just sings on FX. It should have been unsurprising that for every lost competence a new one would be there to be found. Should have been... For #3, judging from some other threads I have read, you will probably get wildly different answers. In my experience, going from 12Mp to 24Mp has involved a minor slowdown - but I must have done a good job configuring my 4y/o iMac for LR because it can't be described as slow, just a bit slower. I have experienced slow processing and what I'm seeing simply is not that. And on top of all that, the D600 IQ just can't be beat.