Just listen to the harmonies.
Those were indeed beautiful harmonies and many groups of that time, including the folk groups were centered around vocal harmonies. I don't know why this has become a lost art in today's music. It was music in its purest form.
My original take on Zep is that their music is really folk music. In that it's under produced, under instrumented, minimalist compared to the pop garbage today. Remember that Zep was pop in it's day.
I agree. Zep was a revolutionary band and constant experimentation was the foundation of everything they (especially Page) did. They brought in all kinds of influences ranging from folk, blues, heavy metal, good ol' rock and roll, Irish ballads even eastern music into the mix. And yes, instead of the constant flat blare of sound that defines much of the music today Zep like the Doors and others had airy depth about their music (even when Page and Bonham were raging) with frequent turns in their songs or changing rhythms. You can't achieve that degree of depth when you have a constant rumbling bass, keyboards, 2 guitars, an over busy drummer and screaming vocals all playing at the same time. Zep always kept it interesting--to say the least.