Perfectly stated. I couldn't agree more.
And that's why, to my ears, Led Zep III is so much heavier than most death or thrash metal, for example. Too many bands lose sense of dynamics (or never ever grasp it at all). The ability to contrast 'light' with 'shade' or bombast with moments of serenity is what ultimately makes heaviness, IMO.
Hence, Led Zep III makes say, Deicide, look like children's music.
Tons of distortion, blast beats and screams to do not equate to heavy music unless they possess that "it", the sense of dynamics which makes good music good.