Now that is interesting, I bever found them catchy except for a few songs, neither Stairway or Kashmir have marked chorusses ... I don't even mean that as a criticism, I thought it was a conscious move to defy traditional song structure in a band that shunned having any of its material released as a single. A lot of Zep's melodies sound esoteric to me, that Eastern, ethno influence ... the almost always extended chords that went beyond root, third and fifth. Their disregard for minor and major keys and harmonies (mixing both) etc. All that is in fact quite original, but catchy?
Do Zep perhaps sound to American to me (and hence their popularity there?). I remember a Rolling Stone review of DP's Burn and it said: "DP still sound as Yuropean as a vampire movie ...". It always made me wonder whether bands like DP or Uriah Heep, which were more successful in Europe than Led Zep, catered more to European, classical music schooled tastes, while Zep with their blues, celtic and ethno ingredients had the recipe for the US market. A valid theory?