Style and lyrical themes
Metallica was influenced by early heavy metal and hard rock bands and artists Black Sabbath, Deep Purple, Kiss, Led Zeppelin, Queen, Ted Nugent, AC/DC, Rush, Aerosmith, Judas Priest, Scorpions and by new wave of British heavy metal (NWOBHM) bands Venom, Motörhead, Saxon, Diamond Head, Blitzkrieg, and Iron Maiden, and early punk rock bands Ramones, Sex Pistols, and the Misfits also influenced Metallica's style as did post-punk band Killing Joke. The band's early releases contained fast tempos, harmonized leads, and nine-minute instrumental tracks. Steve Huey of AllMusic said Ride the Lightning featured "extended, progressive epics; tight, concise groove-rockers". Huey said Metallica expanded its compositional technique and range of expression to take on a more aggressive approach in following releases, and lyrics dealt with personal and socially conscious issues. Religious and military leaders, rage, insanity, monsters, and drugs—among other themes—were explored on Master of Puppets.
In 1991, Huey said Metallica with new producer Bob Rock simplified and streamlined its music for a more commercial approach to appeal to mainstream audiences. Robert Palmer of Rolling Stone said the band abandoned its aggressive, fast tempos to expand its music and expressive range. The change in direction proved commercially successful; Metallica was the band's first album to peak at number one on the Billboard 200. Metallica noticed changes to the rock scene created by the grunge movement of the early 1990s. In Load—an album that has been described as having "an almost alternative rock" approach—the band changed musical direction and focused on non-metal influences. Metallica's new lyrical approach moved away from drugs and monsters, and focused on anger, loss, and retribution. Some fans and critics were not pleased with this change, which included haircuts, the cover art of Load, and headlining the Lollapalooza festival of 1996. David Fricke of Rolling Stone described the move as "goodbye to the moldy stricture and dead-end Puritanism of no-frills thrash", and called Load the heaviest record of 1996. With the release of ReLoad in 1997, the band displayed blues and early hard rock influences, incorporating more rhythm and harmony in song structures.
St. Anger marked another large change in the band's sound. Guitar solos were excluded from the album, leaving a "raw and unpolished sound". The band used drop C tuning; Ulrich's snare drum received particular criticism. New York Magazine's Ethan Brown said it "reverberates with a thwong". The album's lyrics deal with Hetfield's drug rehabilitation and include references to the devil, anti-drug themes, claustrophobia, impending doom, and religious hypocrisy. At the advice of producer Rick Rubin, for its ninth studio album Death Magnetic, the band returned to standard tuning and guitar solos. As a return to Metallica's thrash roots, Death Magnetic was a riff-oriented album featuring intense guitar solos and subtle lyrics dealing with suicide and redemption.