"Nothing Else Matters", a Power Ballad that holds the title as the only love song Metallica has ever put out. It reached number 11 on the Billboard mainstream rock tracks and is still frequently played on rock radio stations.
"The Unforgiven II." Released as a single and had a music video made for it, and played on the radio regularly. Only ever played live once, some time in '97.
In a 2009 interview, Hetfield was asked about the Load period. He revealed that Lars and Kirk were largely responsible for their image change starting with Load. He accordingly hated the cover art (which he labeled a "piss-take" and said Lars and Kirk went with it to annoy him), Anton Corbijn's photos (he said he rejected at least half of them, and said the ones where Lars and Kirk were kissing were intended to annoy him), and agreed that they were musically "unsure" in that period.
He also doesn't like "Don't Tread on Me", and in the infamous Playboy interview of 2001 also expressed disappointment with "Of Wolf and Man" and "Through the Never", calling them "a little wacky".
St. Anger got this too, though they said it was necessary due to... see the Music tab.
Documented in Some Kind of Monster, with St. Anger as its testament.
Their fourth album, ...And Justice For All, is possibly their most aggressive. It was their first album after original bassist Cliff Burton was killed in a bus accident; his replacement, Jason Newsted, was pretty much mixed out of the album altogether.
Their 1984 ballad, "Fade to Black", was written after the band's equipment was stolen after a concert in Boston, including James' prized and rare Marshall amplifier.
Which his mom gave him soon before she died of cancer. His mother's death has been a key inspiration to much of his music, most prominently in "The God That Failed" from Metallica, and "Mama Said" and "Until It Sleeps" from Load.
Defictionalization: The final level of the game Guitar Hero: Metallica (published in early 2009) features the band playing in Antarctica. In December of 2013 Metallica actually would perform a concert in Antarctica.
Fan Nickname: The Four Horsemen, which comes from their song of the same name.
Money, Dear Boy: One of the reasons for The Black Album and the band changing their style to appeal to a bigger audience. While they may have wanted to stick to thrash and progressive and keep their fan base small, eventually making sure they would keep eating and continue to have roofs over their heads became more important. The band have however disputed that this was their main reason for the change: James and Lars were quite vocal about their dissatisfaction with how long the songs on ...And Justice for All were and how the audience responded to them live, with James mentioning in an interview that the band had on previous albums deliberately ramped up the intricate songwriting and multiple sections in response to their own "musical insecurity".
Kirk Hammett is a major horror affectionado, with a huge collection of toys, posters, and miscellanious memorabilia. He seems to be just a general nerd as well, as a fan of both manga and Western comics. Just look at this◊ picture of him!
Streisand Effect: Lars Ulrich's attempts to shut down Napster actually encouraged many people to illegally download Metallica songs, even people who didn't like the music still downloaded it just to spite Lars.
Throw It In: "(Anesthesia) Pulling Teeth" starts with Burton calmly announcing "Bass solo, take 1".
Before he died, Cliff Burton was leading the band into a progressive direction. One can only wonder how far they would have gone into Progressive Metal had Cliff not died.
Also, on S&M, both "No Leaf Clover" and "-Human" had a unique sound to them. They were still rock, like Load and ReLoad, but without the blues and country elements. They were a lot heavier, more technical and had a darker, almost hopeless atmosphere to them. As with the example above, they may have continued in this more musically mature and progressive direction if not for their subsequent Creator Breakdown.
Les Claypool, a personal friend of Kirk Hammett, once auditioned to be Metallica's newest bassist after Burton's death, but he was turned down, because, according to Hetfield, "he was too good" and that he should "do his own thing". Claypool would later form Primus.
James Hetfield had asked John Bush of Armored Saint (later for Anthrax to take over for lead vocals, even after Kill 'Em All was recorded and released. Bush declined, wanting to stay with Armored Saint, who were comprised of childhood friends. Thirty years later he did join them briefly on stage to sing "The Four Horsemen".