As a D700/D300 shooter I have a huge problem with the missing dedicated AF-On button.
I shoot almost exclusively AF-C mode. My experience is that AF-C in focus priority mode delivers substantially more consistent shots of tough subjects like birds in flight.
In order to shoot AF-C focus priority, and in order to be able to do a focus/recompose, I *need* two buttons- AF-ON and an AF Lock button.
If is possible, of course, to give up AF-On and use the shutter button and the one AF-Lock button provided, but there are very good reasons for the AF-On button and I've shot this way for about 7 years now, with the D2H, D200, D200 and D700.
Alternately I can shoot release priority or focus/release, but then, in exchange for nearly $3000 after all the add-ons, I am giving up image quality consistency. Not exiting at all.
I'm not excited, to say the least, about being forced to make a fundamental and to me very undesirable change in the way I shoot, at a very basic level. By my way of thinking, those two buttons are not minor conveniences or some obscure feature. They are as fundamental to me as the shutter button.
I would seriously miss the 8fps I get with the D300/D700 and for that reason alone I need to keep my D700 alive for the foreseeable future. Fpr me 8fps is a specialty tool that I *need* only for certain very specialized subjects. But that Af-On button essentially affects every single image I shoot.
I think Nikon left me out in the dust on this one. I'm not sure what I'm going to do and I'm not terribly excited about paying another $1000 for that button, in the form of a D800.
I also appreciate the 51 AF points on the D300/D700, et al. It's tough to go backwards there too. But that point has already been discussed here.
I have trouble with the idea of a $2100 "entry level camera". Whatever the D600 is, it isn't priced anywhere near my idea of entry level.
I may have a rather niche problem with that camera. I don't think a lot of people shoot AF-C focus priority like I do, although I've never understood why more people don't.