Interesting...
I must say, the song is very nice, and I am extremely glad you put some of your theory work in the commentary too so I have something to go on other than just my ears.
I do have a considerable selection of feedback for you on it, so, as I say to most, please take it "cum grano salis", "with a grain of salt".
The first thing I realized as I listened was how dry the mix was. I'm not sure how familiar you are with mixing and mastering, but neither is very well done. The harp, pizzicato plucks around 4:40ish and several other instruments, and much of the brass is asking for more spreading. Reverb is not very clear... a touch of reverb transforms a piece. Especially make sure your lower ranges have a bit of reverb (be careful not to overdo this, however, as a "muddy bassline" may occur).
As for the thematic and symbolic structure of the piece, I do think you did a good job, but as mentioned before, it's a very mechanical composition for the most part... I have nothing against contemporary style, but I would have liked to see a bit more diversity in the piece. It goes through several stages, as I said above, of just flat action... nothing is changing, it's exposition. That is perhaps what lends it that sense of mechanical composition, but if you are looking to write an expressive and symbolic piece, rule number one is everything is always in motion, like a troubled mind or the summer breeze. You did a good job with this around 3:00.
My main tip regarding that sort of thing is that you should consider trying to add more counter-melodies as parts continue. Since you have decided to work with the concept of repetition building into a "Tapestry of Music", parts weaving in and out to form a greater piece (a very contemporary-minimalist idea, to be honest, which is pretty neat), you need to use the motif and counterpoint to your advantage. These two elements will greatly help your melody evolve. An example of this would be to, in the area of around 0:45, add rapid arpeggio lines for the violins going in and out of the chord.
I liked the subtle use of dissonance throughout the start of the piece, as it lends nicely to the growth of the piece.
You used percussion well, although some more metal percussion would aid the original thematic idea, such as orchestral chimes (imagine it outlining the chords in the 2:00 area with whole notes), waterphone (a lovely and mysterious instrument for the harsh start), and anvil (would be nice to accent the brass hits). Violin harmonics (the tonic perhaps?) in long legato suspensions (several measures) during that beginning would also be nice to hear, as that gives an excellent ominous feel.
You have a good concept of writing an effective melody, but you don't do a lot with counter-melodies except in the area around 3:00 to 4:30ish (perhaps my favorite part, to be honest). Chording seems fine in general, I love using vi chords for mysterious parts, and your use of non-key chords (major VII, minor i, iv minor) is irregular, but works fairly well... although it seems like you're not writing in the key you state it as (B?). However, I give you praise for writing in a sharp key... most composers, albeit those who love strings greatly or those in Broadway composing, write in flat keys.
I could offer many suggestions on where to take instruments for improved melody work, but my review is long enough as it is, and I feel you know what you're doing well enough that it would just be a pile of "maybe ifs".
As a last word, I'd like to thank you for your blurb on defining orchestral music... I agree 100%. Orchestral/classical composition is based heavily upon the ear and mind- what can you do to make this feel or this idea come across?
If you are ever interested in collaborating, I write in a similar style and would be glad to work with you, although I use a smaller range of tools (you seem to have quite the expensive set there), working on an old copy of Finale and a whole set of EWQL VIs I recently purchased.
Anyway, great piece, best of luck!
-Samulis