FF gets you twice the SNR at a given sensor tech, because of

hysics:. Subtracting losses due to sensor area covered by ADCs and readout hardware, you will get a stop or so extra high ISO.
You get a bit less apparent depth of field due to the reduced magnification, and lens performance varies (often more vignetting and worse corner sharpness, but better overall resolution) again

hysics:
That's basically it from a sensor perspective. The bodies they're housed in may get you more (though in the case of a D300/probable D400, it isn't much and will likely be even less). How much it's worth depends on how much you have and what you need. For me, it isn't and likely won't be until I'm a fat cat with my own caribbean island (I may be exaggerating slightly). But a 70-200 was. For someone else, a 70-200 means nothing, where the FF stuff is what they want.
Others may justify their purchases with 'FF magic' or some other tripe. Let them, it makes them happy. Doesn't make it true though.
The 5D II has terrible autofocus compared to the D700, fine. But if you don't need the complex autofocus, that's a lot of useless hardware. On the other hand, the 5dII gets you gobs of resolution. If you don't need that, it's a lot of useless hardware.
Some people get themselves emotionally invested in the technology and equipment rather than output, some exclusively in output, some mix both. Understand yourself and where you fall along that line, you will be much happier for it. FWIW, I mix both. I'm an engineering nerd, but I like pretty things :mrgreen: