When I was a kid I was sure that I was going to hell for listening to this album, very ethereal and transcending. A great beat and the haunting voice of Robert Plant. This song is very blues sounding with a strong lead guitar riff.
Of course I am very biased since Led Zep. was a major band when I was in H.S. I am a guitar/bass musician myself and I tend to judge a song by the style and quality of the instruments. Though on this song you hear Robert Plant lead singer who normally has a heavy rock and roll voice is very moderate (no heavy screaming and other effects) and sets this song apart from songs like "Whole lot of love". He sings very well in general but I like his moderate style on this track.
The guitar lead and slide style is incredible by Jimmy Page and it is mixed A+ with the rest of the song it really stands out.
Overall listen to the preview which is just a hint on how well this song was composed.