One thing I noticed is that Led Zeppelin is a very "physical" band not just in Plant's lyrics, but a lot of it has to do with the rhythm of the music as well. This song is a good example of that. It goes from a simple caress on the cheek to some rough playing in a matter of seconds.
When I heard this for the firs time, I was just a little metalhead, and when the song began i thought : "Oh no, this is smooth, this will be borring". Then came the chorus with this powerfull riff and I was jumped all over the street !