1. Does it avoid the obvious?

2. Do surprises or unexpected events not only make sense in retrospect but
give a feeling of revelation or "aha"?

3. Is the complexity level high compared to pieces in a similar style, yet
it is easy to hear how each event fits into the overall plan?

4. Is the level of psychological or emotional intensity high, does the
piece come out and "grab" you?

5. Is every note necessary and sufficient?

6. Is one left with the impression of superior craftsmanship?

I would suggest that 4 of the 6 are necessary to call it a good piece.
The discussion of Vivaldi concerti probably caused me to think about this.
Let me know if you think I left something out.

Chris Bonds