Structured improvisation rather than improvisated structure, but first emphasis on melody rather than rhythm.
Most metalheads are unable to conceive an expressive melody, most are used to attach chords on preestablished patterns, as if they by themselves were enough to bring sense. Remove this, and songs can't support themselves, because they weren't enough melodius on first place.
There's no authentic connection with classical music if melodies aren't able to be beautifully sung.
Improvisation is evocation, and Metal musicians need a deep internalization of melody (melodies) in order to make coherent improvisations.