After recently discovering I like pistachios I've had a chance to test out the question of how much of it was due to the quality of the pistachios, by trying three levels of pistachio quality. First, the Wonderful Pistachios which are top quality; second, bulk Kirkland pistachios sold at CostCo at a considerably lower price; and third, remaindered, and possibly slightly stale, pistachios at a cut rate at Big Lots.
My conclusions are that I like all of them, but I can definitely tell the different quality levels apart. I probably won't buy the Big Lots ones again because they're not enough cheaper to be worth the difference. The Kirkland ones are perfectly adequate, however, even if I can tell how the Wonderfuls are better.
I still can't tell how much of the difference is because of the overall quality difference between now and when I was a child. Back then, pistachios were polluted with awful green dye, and probably the best of them was worse in terms of freshness than the Big Lots ones today, so quality might still be a more important factor than the changing of my own tastes. I suppose there's no way to know, short of time travel. (And if I had time travel I'd have bigger mysteries than that to resolve!)