What I like most of all is an interesting performance, one that surprises or enlightens. Many professional ones I don't find very interesting. I have spent many years listening to students perform and perhaps this is why I enjoy hearing someone making a more or less successful attempt to play a piece of music despite some obvious limitations. Let's see if we can find some examples. Here is a very professional, very career-oriented, young guitarist named Milos Karadaglic playing the Spanish Romance on television. The performance starts at around 3:40 if you want to skip the interview.
Here is another performance. You might say, no way this is an amateur, and indeed, he is a professional guitar teacher and performer. However, he is not a touring virtuoso, so I hope he will forgive me! This is a lovely performance and shows a real delicacy especially with regard to the dynamics. Nice use of harmonics for the transitions.
Here is another kind of performance, one that shows the problems that can befall an amateur player.
There's something very odd about that rhythm, isn't there? It has a real Cuban sound! What he is doing is leaving out the last note of the triplet on the third beat of the measure so we get a 123 123 12 pattern. Then he adds a little gratuitous vamping. There are a couple of wrong notes in the melody as well.
I think we might see the same kinds of differences with pianists as well. Here is an excellent performance of the first prelude, in C major, from the Well-Tempered Clavier by J. S. Bach. The player is Friedrich Gulda:
Here is an amateur, playing the piece through at home. I think there is a wrong note around 1:54. But I enjoyed listening to this because there was a slight sense of surprise at each new harmony. The problem with a very seasoned professional like Gulda is that he knows the piece so well, there are no surprises.
One of the great Bach players is Glenn Gould. Here he is with the same piece:
There is a sense of exploration and savoring about that performance. How about this one?
That is more the kind of professional performance that I often find dull. It is perhaps too smooth and perfect with no sense of exploring a kaleidoscope of harmonies. Now here is a very interesting amateur performance:
Interestingly 'crunchy' in places and with some unpredictable ritards. The problem with professional performers is that they can be so consistent that their musical gestures become predictable and therefore dull. Amateurs can often surprise you. But of course, they can be often inaccurate and make the wrong musical choices.
So, as I say, I am of two minds about professional versus amateur performers...