It takes time to really appreciate Mozart. Sure, you can like Mozart's music because it's pretty, and you can even like Mozart's music because, on the surface, it's beautiful.In this world of increasing commodification when it comes to things musical it is easy to forget what music (particularly the "classical" kind from the actual Classical Period) does for us as musicians. It allows us to be eloquent, or at least to make an attempt at being eloquent.
The thing about Mozart that makes his music great is the way it makes musicians behave. It makes them listen to one another in new ways, and in the case of pianists, it makes them listen to themselves in new ways. It makes non pianists take care of their intonation and the clarity of their sound, and it makes them strive to be reverent and tasteful while pouring their hearts out.
Sometimes it is difficult, when listening to recordings, to remember that music making is a highly spontaneous activity that is disguised as a scripted activity. What is amazing about Mozart is how, in the hands of great musicians, by simply following the pitches, rhythms, dynamics, and articulations, the music making can go immediately to a "place" that is beyond the normal. Mozart is what makes us grow as musicians. As Rossini put it (and I'm quoting approximately), "Mozart was inspiration in my youth, the envy of my adulthood, and the comfort of my old age."
A great performance of the slow movement of the "Dissonance" Quartet can be an almost transcendental experience because the music compels the musicians to reach beyond themselves. Musicians learn from Mozart, and Mozart keeps teaching. He elevates our expectations from music by other composers, and teaches us to appreciate the wholeness of natural expression.
Listen to Dinu Lipati play Mozart on the piano, and notice how Mozart brings out the best in him.
In Mozart we have a composer who could write music that always feels good to play, no matter how accomplished a player you happen to be. The better you can play, the more you get out of Mozart. Playing Mozart allows us to step, whether individually or collectively, out of our usual palette of musical colors, and find a special kind of emotional stimulation that challenges us to be more honest, play more in tune, play with cleaner articulation, play with a more singing sound, and to give every phrase its place in the hierarchy of a given piece.
Mozart sounds new because his music has a purely expressive purpose, and that expressive purpose comes out differently for all musicians, even when we follow all the "directions" to the letter. That individual expression happens wherever you find Mozart. It even happens when I play his music (with my sad lack of any technique) on the piano, and it happens when a teenage wind player first learns a Mozart concerto. It happens in Mozart's early string quartets, and it happens (sometimes more easily, due to his incorporation of Haydn's innovations) in his later string quartets. It happens when a group of young musicians play his music (with non-standard instrumentation) on top of a dump in Paraguay, or when a great orchestra plays the Jupiter Symphony for the hundredth time.
Here's Dinu Lipati playing Mozart's 8th Piano Sonata in A minor:
This meeting of the musical minds is what I believe we should all aspire towards, from the perspective of a composer as well as from the perspective of a performing musician playing any instrument.