I love the orchestration, first and foremost. There is a sense of going places, merely through the repeated progression of notes in the strings, ascending until the end of a verse, where the full orchestra comes in and notes descend together.
Robert Plant's vocals are, of course, another key element of the song and, along with them, the lyrics. SongMeanings.net always has some hilarious interpretations posted by random people on the interwebs, and Kashmir is no exception; everyone on the site is arguing whether this song is about Africa or Tolkien, and also whether P. Diddy wrote it. (It's about the desert, and P. Diddy raps over the orchestration in his song Come With Me. Sorry, elf fans and crazy people.)
Anyways, the lyrics vie with the orchestration for the most beautiful part of the song. I think they're poetry:
Oh, pilot of the storm who leaves no trace, like thoughts inside a dream
Heed the path that led me to that place, yellow desert stream
My Shangri-La beneath the summer moon, I will return again
Sure as the dust that floats high in June, when movin' through Kashmir.
In a discussion of best Zeppelin songs, there are many to be mentioned. Stairway to Heaven, Heartbreaker, and When the Levee Breaks are usually in the discussion. (Though when I asked Spousal Unit his favorite, he said, "Trick question. Led Zeppelin's music library is all one song." It could well be; this is akin to picking top Beatles songs.) This top ten list puts When the Levee Breaks first, and then forgets to add number 10. Another list also puts When the Levee Breaks at the top, while Kashmir ranks fourth, behind Stairway.
My true favorite may change over time, but for now, I view Kashmir as their most well-rounded tune - lyrically, musically, and in the emotion behind it, too.
And to close, another of my favorite zeppelin moments.