This is a great thread. It's nice to see a vocalist with so much knowledge about composition. I'll toss in a contirbution along with the kudos. A great song and a really simple example of some of what Arnaud is talking about composition wise is a song from The Abscence - "Dead and Gone". It shows how more stripped down metal can use concepts like themes etc.
http://youtu.be/ZYIj7eDz8X0
The intro riff is the theme. It's the singular and most prominent part of the song in the beginning. At :29 They move into a chord based structure. The riff is still actually playing behind it, but has taken a supportive role and is keeping the sense of motion going. The chords are more rythmically tied to the vocalists pace, creating a sense of space without dragging the verses down.
At :39 The guitars move to a harmonized melody in the lead. Again, the riff is still moving underneath as a supportive piece. The harmonized melody is setting the stage for the upcoming chorus melody that will temporarily depart from the use of the motif of the intro riff.
At 1:10 the riff returns in an altered form using just the first couple of notes in repetition along with a bass movement to create a tension and move the song back to the theme completely.
1:24 The theme riff returns, but is now being used with a different rhythmic sense adding tension, and a sense of the song evolving even though the same motif is playing again.
2:18, again the theme is moved to a supporting role and a new chord movement added over it.
2:32 the riff becomes the feature again, but the bass is moved off of it's former unison role to indicate a coming change in the song and helps create a more gradual transition.
There's much more to analyze in the song, but it's a good simple example of how even less complex, and non symphonic music can make use of the concepts Arnaud is talking about. Well done man!