Physical Graffiti was a masterpiece.
Eh. I've heard plenty of better albums and double albums. And Led Zeppelin albums, for that matter.
-- LZ has a reputation as a loud band but their early albums were fun and inventive with varying dynamics, tempo changes, and different riffs and grooves throughout songs to keep them interesting. Later LZ had a bad habit of taking one riff and one tempo and just pounding it at you forever.
Physical Graffiti isn't nearly as bad about this as their late-period albums, but it isn't consistently fun like
Houses of the Holy.
-- About 1/3rd of the songs of the albums were the scraps/leftovers from earlier recording projects. I wish I wish I wish "Houses of the Holy" was on the album by the same name. It's already an excellent album; knock off "The Crunge" or "D'yer Mak'er" and oh boy. While that is an exception, you can kind of tell parts of
Physical Graffiti are a collection of would-be B-sides (if LZ had singles) thrown in there.
-- The production just isn't as crisp and biting as the earlier albums. The guitar and drums especially just don't hit like they on earlier stuff like, say, "Good Times Bad Times" with its epic crunchiness.
-- Plant was losing (or had lost!) his upper register by 1975. He just couldn't hit the notes like he could in 1970. This is when the accusation that his voice is "whiny" really starts to become true.
It's just the point where their inventiveness and majesty gives way to pretention and bombast.