Now, keeping in mind that GPO is the one we get the most performances right now, I had an epiphone!
This question seems pretty cut and dry, but I realized the answer is not so simple. I bet everyone says right away, "The audience! Duh!"
Think about this, though. Who decides what music even get's presented the audience we write for? The performers! These are who we have to impress first every time!
Now Phillip Glass's music is actually well liked by his fans, but performers hate it. (Oh god no! Not the same phrase over and over again for 10 mintues! AHHH!) So Glass hires his own ensemble to perform his music. We can't do that, that is why we have GPO.
What's the difference between impressing an audience and a performer? Well most all of us here are performers, so we should know. For serious professional performers there are two criteria to what they play. First, they have to like the music personally. Second, and just as important ... the act of performing it must be interesting to them if not challenging.
This doesn't mean the music has to be hard... just challenging. Even Adagio for Strings by Barber sounds easy, but it covers a wide range on the instruments and getting the proper expressivness is difficult.
So, in conclusion, I think we could all do well to think about whether we are writing something that might catch a performer's eye on the music shelf, the audience's love will follow.
As a matter of fact I now want to write something for the trombone!! I haven't played it in years because I switched to piano in college...
.... must ... finish ... the .... flute .... sonata!!!