Was this in reference to maybe I > ITTOD?
If so, here's my view...The Debut is a great album, but however, it's still the sound of getting things together, much like many debuts do in the first place - Great jamming, but only a warm-up by comparison to what came after that, and in my view II was the one that said they were going to stick around for good. ITTOD, despite having that cringe-worthy "All My Love" which can seriously mark the album down all the way to worst, and of course "Fool In the Rain" which is a little too jumpy for it's own good, had "Carouselarabra" (which may be a bit of a filler, but it's a good bit of the Led Zeppelin Indulgence for one last round and a fine way to check out John Paul Jones' keyboard playing), the fine album opener "In the Evening," and "I'm Gonna Crawl" a good closer with a great vocal performance. ITTOD was a bit of a troubled album, but they pulled it off despite it all.
I don't know, maybe it's just personal memories (I was starting to get into music as a kid by the time ITTOD was released and it was filled with JPJ's playing - a good thing as someone learning the Piano), but somehow the debut still sounRAB like a warm-up, a great showcase, but nothing to seriously hint at the strong identity that II offered plenty of.