When I was a kid, this was the second most popular Zep record (behind the fourth one). I can see why, given that both sides start off with utterly tremendous riff-rockers (especially considering that the first record had side-openers that were the weakest cuts on the side), but in the context of the times, I can't help but find this a step back from the novelty and freshness of their debut platter, and showed them in serious danger of becoming just another pedestrian hard-rock band.
Of course, that didn't happen and they made a surprising left-turn into folk-rock on their third record, before managing to synthesize all the different aspects of their sound in an honestly pretty masterful way on their fourth, but if I'd heard the first Zep record, maybe even seen their incredible live performances, and then eagerly awaited their next record, I would've probably heard it not as some kind of definitive statement of their early sound but as a disappointing sophomore outing.
The big change, I guess, is that they were actually making some sort of attempt at writing original songs. "What Is And What Should Never Be" is possibly even an improvement on the soft-loud dynamics of their cover of Anne Bredon's "Babe I'm Gonna Leave You", and while "Ramble On", the first of their hobbit jams, may actually be the least of them, that's only because they're all of them tremendous pieces of songcraft, and you know, one of them has to be "worst". Pretty sure that the reason Zep didn't get more stick for writing songs about hobbits is because all of their Middle Earth songs were actually incredibly awesome and you don't even mind the utter unlikelihood that Gollum did, in fact, steal Robert Plant's chick.
That's the good bit. As for the riff-rockers, they're undeniably classics, but you know what, if all Zeppelin was good for was writing riff-rockers there wouldn't be any difference between them and, say, Thin Lizzy. Furthermore, "Heartbreaker" suffers from slamming right into what is perhaps one of the worst songs in the entire Led Zeppelin canon, "Living Loving Maid"... one of the great things about the release of that 1990 box set was that idiot DJs realized that "Heartbreaker" and "Living Loving Maid" were entirely different songs and didn't HAVE to be played back to back. (Songs like "Kashmir" got surprisingly little love during the '80s, I remember.) Anyway, just a few more seconds of silence between the songs might have mitigated the pain, but that's just quibbling- the real problem is that the song exists at all.
As for "Whole Lotta Love", it has this very weird late '60s psychedelic guff thing surrounding it, kind of a Chambers Brothers thing. I am not sure I would ever view Zep as being a "psychedelic" band in the purest sense, but then again there is that whole freak-out middle section and outro... is it weird that Zep doing psychedelia should sound as alien as Zep doing James Brown? I guess it's a credit to the times, or the riff, or something, that the song continues to be held in such high esteem despite the goofball middle section. Not to say that I don't LIKE the middle section, or that their use of theremin wasn't totally awesome, but in terms of songcraft and construction I wouldn't exactly put it up there with "Good Vibrations", you know?
And then there is the fact that the song is blatantly about dick. In the annals of rock history, few records have been made by men so fixated on their own penises ("Fillmore East June 1971" comes to mind, but that record is, shall we say, heavily indebted to Zeppelin's pioneering penis-hagiography work.) Not only is there "Whole Lotta Love", but the record also features Plant moaning "Squeeze my lemon till the juice runs down my leg", and then there is the drum solo, "Moby Dick", which you can try to tell me is about a man's desperate quest to defy the gods in the name of revenge, but you know what, I'm going to say that one comes back to penis as well. (Despite being the home of a nearly infinitely prolonged drum solo in concert, I do have to give props to the riff on Moby Dick as well, which, were it to accompany anything besides a drum solo, might perhaps hold up as well as "Whole Lotta Love" or "Heartbreaker"- though "The Girl I Love" does somewhat argue against that theory.)
Then there is "Thank You", a tender love song that suffers slightly both from being sung by a man whose voice positively oozes insincerity and from the context of, you know, being surrounded by songs about dick.
Also on the record are a couple of blues reworkings- "Lemon Song" tips its jimmy hat to "Killing Floor" and to Robert Johnson (although not nearly as well as the 1969 BBC recording "Traveling Riverside Blues" did), and there's another Willie Dixon cover in "Bring It On Home", though it can't hope to measure up to either of their Willie Dixon covers from the first record. Not that either of the blues cuts are BAD songs, far from it, but they seem to show a band stagnating, rather than progressing- getting worse, and not better. Nowadays it's obvious that their best days were still ahead of them at this point, but without the benefit of hindsight that claim is more difficult to make.