I've always thought it was cool that only one Led Zeppelin song features a singer from outside the band: "The Battle of Evermore" from IV (1971) is a duet sung by Robert Plant and Sandy Denny, the great folk-rock singer who had a prolific career as a solo artist and with Fairport Convention. The song was written quickly according to Page, who said that its composition began with the first time that he tried a mandolin. Moments like this one are the reason for the thinking that some musicians follow, where the belief is that the sound will lead the player, not the other way around. It certainly worked in this case. It's a rich, dramatic cut that combines the blues with Tolkien (and other folk-tale elements), and without the use of electric guitars or drums is not out of place on an album that includes "When the Levee Breaks" (for example). A lot of the credit has to go to Plant and Denny, who create an other-worldly atmosphere with their vocals. Check it out: http://www.youtube.com/watch?v=uTuzVZsKAhY

Led Zeppelin's version of "Babe, I'm Gonna Leave You" from their 1969 debut album (written by Anne Bredon and discovered by Page and Plant on Joan Baez in Concert, Part One from 1962) is very interesting. Musically, it provides an excellent cross-section of the Led Zeppelin sound - the folk and blues influences, the acoustic and electric guitars, the dynamic ranges of all four members - it's all there. It also gives a fine example of how they put as much effort (and originality, strange as it may sound) into their versions of non-original material as they did into their own. Listen to the two versions (they're on YouTube), and you'll hear what I mean.

Their treatment of the lyric is also worth noting, because the protagonist is shown struggling with a decision, seeming to make up his mind, only to reverse it shortly thereafter. Some commentators have credited Shakespeare as being the first artist to show characters doing this - to literally show characters thinking during a story. (Before him, characters spoke in a style that is best described as declamatory - in other words, they simply announced their situation and/or mindset directly, with little or no second thought. Come to think of it, there are still plenty of stories in books and on film told that way. And although this style may be superficially entertaining, there are two problems with it: 1. It's unrealistic, because it's not the way people are. 2. It's boring, because it leaves the audience with nothing to think about.) Back to the song. My main point in all of this is that the song is actually the reverse of the common situation - on the surface, it's a simple story, but it isn't really, because it shows a human mind at work.

Led Zeppelin was certainly one of the most misunderstood bands. For one thing, the group was comprised of professional studio musicians. (The photo on the back cover of the first album is more representative of what the members were really like than any other that I've seen. The image that grew around them was more representative of the time and their success than reality.) In listening to their music, one can hear very clearly the professionalism and musicianship that went into the recordings. For an example, listen to the bass lines in "Stairway to Heaven", or to the opening track ("Good Times, Bad Times") on the aforementioned first album: It is one of the most sophisticated studio performances by a hard rock band ever. Compare it with tracks by other bands of the same time and genre, like Blue Cheer, for instance. And this was their starting point, remember.
But like all serious musicians, the band's primary interest became learning and improving. Therefore, experimentation and long passages of improvisation became integral to their work. All four members participated, including Robert Plant, the vocalist. The interplay between Page and Plant became one of the band's innovations. (It could be argued that no other group got so much out of their singer.) Also, their music was based so completely on the blues that some have accused them of plagiarism. (Eliot's line about bad poets imitating as opposed to good poets stealing comes to mind.) But it was a respectful, almost scholarly approach to the blues, not a haphazard one. What resulted from all of this was a group that was never formulaic - their albums and songs display amazing diversity - whose songwriting and studio mastery rivaled anyone's, including The Beatles. As supporting evidence, I recommend giving "The Rain Song" a listen. (To learn more about their music, In the Houses of the Holy: Led Zeppelin and the Power of Rock Music by the musicologist Susan Fast is a must.)