These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Big Lipped Alligator Moment: "Supernaut" starts off as a typical heavy number before going off, without warning, into a Caribbean flavored acoustic shuffle, then going back to the main riff as if nothing had happened. Despite, or even because of this, it's a favorite among fans.
Covered Up: Not many fans remember who wrote and performed Warning and Evil Woman before Sabbath. For the record, they were Aynsley Dunbar's Retaliation (he is a drummer who played with Frank Zappa, David Bowie, Whitesnake, Journey and many more) and Crow (a late-60s/early-70s band from Minnesota), respectively.
Critical Dissonance: During the 1970s Black Sabbath managed to become very popular among rock fans, but they were generally loathed by critics.
Dork Age: Technical Ecstasy and Never Say Die consist mostly of typical 1970s rock with lots of keyboards and very few metal elements. Indeed, the only song off either album the band still plays today is "Dirty Women" from Technical Ecstasy. The only on-stage acknowledgement Never Say Die gets is that the flight mask from the album cover is sometimes on the bass drums. The mask also frequently appears in promotional materials and merch.
Epic Riff: So many. "Iron Man", "Paranoid", "Sweet Leaf", "Into the Void", "Children of the Grave", "Supernaut"...
Rob Zombie once said that any metal band ever is ripping off of Black Sabbath, because any good metal riff out there, Sabbath wrote it first, either in a slightly different key, or faster, or slower. Most metal guitarists will say this is pretty much true.
There are no albums between Sabotage and Heaven and Hell.
There are no albums after Sabotage, period.
There was no Sabbath when Ozzy left the band.
There are no more albums after Dio left.
There were no albums beforeHeaven and Hell.
Hilarious in Hindsight: The song, "Iron Man," was specifically written in a way to avoid a lawsuit from Marvel Comics with its Iron Man. Today, the song's opening riff is happily used by Marvel Studios as the de facto theme of the superhero. Tony Stark even wears a Black Sabbath T-shirt throughout The Avengers.
Mondegreen: A rather infamous one, the lyrics to "Paranoid" often sound like "I tell you to end your life" when they're actually "I tell you to enjoy life"
In-universe example: Ozzy misheard the lyrics in the original version of Warning and sings "I was born without you" rather than "warned about you".
Nightmare Fuel: While a lot of Sabbath's music sounds tame compared to their successors, "Black Sabbath", "N.I.B.", the first fifty seconds of "Sleeping Village", "Paranoid", "War Pigs", "Hand Of Doom", "Iron Man", "Children of the Grave" and "Lord of This World" hit it out of the park in terms of scary.
Hell, Ian Gillan was probably considered the worst. Not because he was a bad singer, but because he was a really bad match. Deep Purple and Black Sabbath are two very different things.
Stoic Woobie: Tony Iommi. His life sucked when he was a kid. His life still sucked slightly less when he grew up. You will never see him complain about it. Not even in his book, when he talks about his emotionally abusive childhoood with parents that were constantly fighting, watching his beloved Papa (grandfather) die, losing his fingertips, being beaten up by kids in gangs, etc. Then he grows up and gets swindled out of a lot of money, still gets beaten up by adults in gangs, management screws him over repeatedly (see swindling except a little worse), watches his dad die, etc. etc. Then he gets lymphoma. Poor guy.