This has always kind of intrigued me. I have seen many students who are good in academics (the scale I am using for this is GPA, like around 3.8–4.0) and average in PhD research. I mean, they do research, but it's just the kind you would expect from an average PhD—publish dissertation, write to one or two top journals, and then graduate.
I have also seen a few students who barely maintain their GPAs (they usually hang around 3.3–3.5) but are so good and focused in research that they do groundbreaking stuff, and some even have filed and received patents on their dissertation.
My question is: how much of a PhD is about taking courses and excelling them, and how much of a PhD is about excelling in research, and how well are they interconnected? From what I have seen, there seems to be no precise correlation between GPA and quality of research. Is it because PhD research concentrates only on a precise problem? Or is it because of individual motivational factors?