Rubenstein playing Chopin is an easy call. The piece is written to be very expressive--so much so that almost any musically sensitive player will capture at least some of it. But listen to the subtleties: the eighth-note pulses are all slightly different in length. Some are extended slightly for emphasis, others passed over slightly to create forward movement. Each note is handled with a slightly different touch, some stronger, some lighter. And listen to the piano of the second phrase, how delicate it is compared to the first phrase. But more vigorous and motoric music can also be expressive. Here is Horowitz playing Mozart:
Now the sin or vice that is the opposite of this could either be melodrama--emotionalism to soppy excess--or a typical recent failing: industrialized musical product. Let's have an example of each. Here, standing up for melodrama, is the inevitable Andrew Lloyd Webber:
Going in the other direction we have industrialized musical product:
Now it is undoubtedly the case that both these kinds of music have their utilitarian purpose. I just don't think it is primarily an aesthetic one! One other point: what I am talking about here is largely focused on the composition itself, rather than different performances. I think the same principles would apply to different performances as well. I'm sure you could overdo the expression in a piece by Chopin, or play it mechanically.
Oddly enough, looking over this post I see that I have unwittingly followed a principle put forth by Aristotle in his Nicomachean Ethics. In it he follows the principle that the right course of action is a moderate one, between two extremes. For example, courage is a virtue between the two extremes of cowardice and recklessness. Similarly, in my formulation, expressiveness is an aesthetic virtue between the two extremes of melodrama and the mechanical.
I welcome discussion of either the principles or the examples I have chosen. Have at it!