This is probably a goofy idea, but...
I was listening to a live recording of a piano concerto today and I noticed one thing that's missing in the Garritan demos. Between the movements of the live recording I didn't hear silence. I heard the ambient sounds of a concert hall full of people trying to be quiet. At the breaks in the music people adjusted themselves in their seat. cleared their throats, coughed quietly, rustled the pages of their programs, etc.
To get that final step in realism it seems like it would be fun to layer in an audio track of ambient audience sounds. All I could find Googling sound effects was the big sounds a crowd made, cheering, applause, talking, etc. Does anybody know where one might come up with a wave file of the kind of subtle audience noises one hears before and between the pieces of music at a live performance?
Oh, and one more thing. As the last piano chord of the Chopin concerto died out to silence, there was one more subtle sound: The soft, but distinct "thunk" of the piano keys falling back into place as the pianist released the last, now silent chord. That's a nice touch too!