In my response to this topic I am going to slightly rephrase the question : What *makes* a pianist great vs. just good ?
It is tempting to take things like articulation, phrasing, speed etc. and treat them as
causes of greatness, or greatness itself. Then these things that we consider positive get pooled up together. It is tempting to think along these lines : In the "good" pool, there are a certain amount of these positive things, and a certain amount of negative, and a certain level of execution. In the "great" pool, there are a certain amount and mixture of these positive things and a certain amount of negative, and a certain level of execution.
We act as though people subscribe first to the pool of being "good", and that there are a certain amount of pianists swimming within that. They all contain very similar traits because they are swimming in that pool of traits which they have subscribed to. But there are a number of swimmers who are only there because they see it as closer to the "great" pool and secretly enter the lottery to win a lifetime subscription to the pool of "great". If they are lucky, they think, they will win and at some point they will simply jump like a fish from one pool to the other. Once in that pool, they will swim in all the qualities of greatness.
Trying to pool together qualities of performance/musicianship as descriptions of greatness and what
makes greatness, I think is a misguided approach at becoming and discerning greatness. Phrasing, speed, articulation, etc. are not what
makes a person good or great. They are not a
cause but a
result. Like fire. Fire is not a cause, but a result of a balanced mixture of elements. And the interesting thing about fire is, depending on how it is built and attended to, it can get hotter and hotter. What makes it hot though is not simply the heat, but the way it got there.
With pianists/musicians, when we are talking about the difference between "good" and "great", we are not just talking about how hot their fires burn, but
how their fires get to be burning so hot. And that is completely individual in terms of how one deals with the challenge of that endeavor (you can't build a fire inside of a pool

).
I don't think there is a lottery, I don't believe there are any pools, and I don't believe there is any such thing as a lifetime subscription to the pool of greatness. In the end, there is just the individual, the instrument, and a lifetime of moments spent building fires.
m1469
ps- don't burn your instrument (no matter how tempting it may be)
