I see this characterization often in writings about them. The bio on Pandora that came up a few minutes ago went as far as to state "Led Zeppelin was the definitive heavy metal band".
I must admit that my knowledge of "metal" is fairly limited, so perhaps the title would be better as "Led Zeppelin do not fit my idea of metal".
To me, when I think metal, I think bands like Metalica or Godsmack. If people asked me to name a couple of metal songs, the first two that probably would come to mind are "Enter Sandman" or "Whatever". These are bands with overdistorted midrange scooped leads, strongly strummed, palm muted power chords in a simple head banging friendly rhythm, vocals bordering on yelling, an overall dark tone and theme (near ubiquitous use of minor keys) -- well you get the idea. I doubt many people would consider these bands to not be metal, however given my relative lack of knowledge of all things metal maybe my idea of this genre is too narrow, and Zep fits better than I realize.
The only song I can think of that I would classify as metal by Zep is "Immigrant Song," The live version a fair bit moreso than the album version. This song has many of the ingredients I listed for a metal song. Maybe the main thing not there is the scooped mids, but this was not popular until much later.
Certainly, Zep was an important precursor to the metal genre, but I think saying that they actually fall within this genre is a bit of a stretch. Zep probably has more in common with 50s rock than metal. I mean seriously, is "Hot Dog" even remotely metal?
Of course, this is all a matter of opinion. Normally, it wouldn't make that big of a difference. The thing I don't like about the characterization is that many people have a very wrong idea about what Zep sounds like. I'll admit, many years ago before I started enjoying them, I was in this camp. A great many people unfamiliar with the band seem to think that they are an older version of Metallica. Case in point: I was recently on a camping trip and some women asked (borderline begged) me to repeat "Fool in the Rain" on a playlist. I replied that "you don't exactly have to twist my arm to repeat a zeppelin song". The reaction of one of the girls was basically "this is Zeppelin?!?" Certainly not their harder stuff, but it does demonstrate that people not into the band don't really have a good idea of their sound. This is not the only time this has happened either. Most people have only heard "Stairway" which they assume is a power ballad and seem shocked that the rest of their songs don't sound like Metallica or something in that vein. On a side note, the other girl knew who John Bonham was, which is commendable.
