Ive been blown away by that harmonica solo for years, (and the drums) but I think its just a combination of great playing and great production. There are a lot of harmonica players that can get their instrument sounding like a guitar. A few years ago I saw a blues duo called Roy Rogers and Norton Buffalo, and the harmonica playing was a lot like that. The dude sounded almost like Stevie Ray Vaughn on his harmonica, like bending sounds and incredible sustain to the notes. It was very similar to when the levee breaks, but you dont hear it as much on their recordings, because the production is different. The backwards echo adds a lot to Zeppelin's version, no doubt.