No band has ever failed to live up to its own potential quite like Metallica. While the other three quarters of thrash metal's "big four" (Anthrax, Megadeth and Slayer) still cut records and tour for receptive audiences, Metallica moved on to Kid Rock and Nickelback territory. We're used to it now, but it's easy to forget that this was the band that made Kill 'Em All.
The general consensus on Metallica is this: They peaked with Master of Puppets. ...And Justice For All was a bit of a departure, but still a solid record. Then came the self-titled album, followed by the haircuts (Load and ReLoad), hooking up with that guy from Suicidal Tendencies to make a bad Kyuss record (St. Anger), failed attempts at reclaiming past glories (Death Magnetic) and then, well, Loutallica. I have a more heretical view of Metallica's catalog; I think they peaked at Kill 'Em All and I'm not quite sure what happened afterward.
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Metallica's debut is a pitch-perfect foray into the world of heavy metal thunder. The riffs sound like Motörhead played at 45 rpms, Hetfield's vocals a slightly grittier version of NWOBHM outfit Sweet Savage's Ray Haller. While clearly laying the template for the shape of metal to come, Kill 'Em All is, above all else, a very fast and heavy rock and roll record. The debut records of the other "Big Four" bands feature tighter-than-tight, palm-muted, all-down-stroke riff fests, whereas Metallica's freshman effort still has the little shimmy and wiggle that's made people shake their ass since Little Richard, Chuck Berry and Bo Diddley.
Compare the intro of Kill 'Em All to that of Ride The Lightning. The former opens with the bombast of four guys trying to out-loud each during a soundcheck. The latter begins with a little chamber music, an intro that sounds more like harpsichord than rock guitar. It's not that Ride has nothing good to offer. On the contrary, "Creeping Death" stands as one of the band's best efforts not on Kill 'Em All. But can you imagine "Fade To Black" anywhere on their debut?
Now take a look at Master of Puppets: Popular consensus maintains that this is "the best Metallica record." And, indeed, it is an improvement over Ride. "Battery" is a crucial track, while "The Thing That Should Not Be" and "Damage Inc." show the band being heavy and accessible at the same time. They seem on the verge of something really big throughout.