Having looked at each section of 'Stairway to Heaven' in detail, now we can draw conclusions and address why it's one of the greatest achievements in rock history.
My answer to why it's so great is its organic development – the way the melody and harmony of the verses grow out of what came before, continuously blending old material with new material. This graph illustrates:
The x axis indicates the verse # and phrase #, separated by a period (ex: 1.4 means verse 1, phrase 4; 3.1 means verse 3, phrase 1).
The y axis refers to the phrases.
M = melody
H = harmony
So the melody and harmony are together for the first three phrases of the first verse, and the first phrase of the second verse, but they are split for the fourth phrase of the first verse, and all of the third through sixth verses. It is that split that allows for organic development. Because when the harmony uses the new (d) phrase in verse 3, the melody simultaneously keeps the (a) phrase. Then in verse 5, the melody uses the new (e) phrase, while the harmony simultaneously keeps the (d) harmonies. Each step grows out of what came before by incorporating something new while also retaining something old.
In the preface to BEATLESTUDY, volume 1: Structural Analysis of Beatles Music, I wrote:
The fundamental goal for any composer is to keep the material familiar and internally consistent enough so that it clearly belongs together, but different enough to avoid monotony. A succession of unrelated material will appear disjointed and confusing, while a succession of unchanging material will become predictable and boring. Compositional skill is in large part the ability to balance the two. And one of the best ways to strike that balance is through structure.
Indeed, the organic growth and structure of 'Stairway to Heaven' strikes that balance as well as any piece of music I've ever encountered.
Aaron Krerowicz, pop music scholar
An informal but highly analytic study of the music of Led Zeppelin.