I have to agree, the first time I heard Immigrant Song I kind of felt we were in the presence of something new. Not metal but something new.I'd argue that some of their stuff is heavier than conventional heavy metal (live versions of Whole Lotta Love, anyone?), but I'd never consider them a "metal band"... at the end of the day, it's still grooving, I-IV-V based, pentatonic-inflected material.
That being said, the Immigrant Song is notable exception with its thrashing, chugging riff.