I love my 5+ year old D300 ,but to be honest it has never been able to perform all I wanted it too , I notice that almost all replies have been from US members so far and I can well imagine that the amount of good useable (sunny day) light that is available most of the time ,means that one can get the most out of such a camera .
however Living in a small valley in south wales (uk) its gloomy most of the time ,bright sunny days happen for only a couple of months a year , and for 99% of the winter , if I want to take pictures , I am forced to use High ISO , and thats not just to be able to use a smaller aperture or higher speed ,its to barely get a picture at all .
this is not a fault of the camera of course ,its simply a problem of circumstance ,that the light is rarely ample to be able to have a choice with camera adjustments to (nail the shot)
I cannot afford a D4 , I could push to a D800 if I wanted ,but need reach for my choosen hobby (wildlife/birds) and a 300mm with TC can give me excellent results at an equivilent 600mm+ at a modest price .
yes ,full frame models offer crop modes ,but the view in the viewfinder is totally inadequate , try shooting Biffs with a D800 in crop mode with a longish lens , and you will see what i mean ,(and yes, i have tried)
each user probably has thier own reasons for wanting (or not) an upgrade ,and i am sure there are many other reasons , but for me its the poor ISO performance , in poor light, that has me planning an upgrade as soon as a suitable DX becomes available .
its a bit sad that the D300 is still, in my oppinion , the best DX ever made , and with the exception of the sensor performance , still is ,it has everything going for it ,and is wanting only a new high performance sensor to make it "king" again .