Maybe something like this:
The high point in creativity was Greek drama; for these reasons:
1. It was a sucessful combination of poetry, drama, costumes, mime, instrumental music, dance, song, and so on, giving it greater scope than any other art form.
2. The subject matter came from myth, portraying humanity in universal terms.
3. The content had religious signifigance.
4. The entire community took part.
With the introduction of Christianity, a new philosophical direction was taken, and with the emphasis on the afterlife, the value of art diminished. Decay set in, and performance became entertainment instead of art. Wagner wanted to use Greek drama as a model with 19th century resources to improve it.
Whereas in symphonies music has themes, their development, and some kind of form, Wagner's music modulated with the rest of the spectacle. If the character mumbled something sad, the music was sad; if two lines later he expressed hope, the music expressed hope. And so on.
I think this is why I find Wagner's music so boring--if there's a knock at the door, the character may vasilate between answering or not, and each moment of indecision must be described musically, and change with the "yes I will/no I won't" behavior. On stage it may be so perfectly integrated that an audience only experiences the neat package. On a stand-alone basis, though, the music can be dreary and intolerable. I'll bet those arguing otherwise cannot separate the music from the rest in their mind.
It seems ironic--after all, people attend opera for the music, certainly not the plots. Given the choice, I'll pass on Wagner and choose Gilbert and Sullivan.
Thinking is overrated