Big Lipped Alligator Moment: Dazzler's introduction during the first third of the arc feels jarringly out-of-place. She made up for it in later appearances, but in the context of this story she's very much this.
John Byrne's artistic contributions can't be discounted either. His art was at turns incredibly complex and deeply evocative; the book is still held up as a standard for superhero comics today thanks to their collaboration.
This storyline is also generally considered the point that Wolverine started to become the superstar he's become. Specifically, the point where he re-infiltrates the Hellfire Club and takes the Club floor by floor is the turning point where Logan transformed into the badass he is today.
Evil Is Sexy: The White Queen. The Black Queen. Dark Phoenix herself, for some. And for Jean in-story, Jason Wyngarde.
It Was His Sled: Jean Grey's death in this is infamous, though even the creators themselves didn't know how it would turn out until they wrote it.
Even more so, at one point, the mysterious Jason Wyngarde, the man who was slowly and carefully setting up the More Than Mind Control that he hoped would bring the Phoenix under his control, is dramatically revealed as none other than old-school enemy Mastermind. At the time, Mastermind had only been seen as a recurring Rogues Gallery member back before the end of the original 1960s run, and we never saw any hint of his outside life, let alone gotten a civilian name. By now, both names have been used for him for so long that for anyone who didn't begin with the original sixties run and read in order, the revelation comes off as being told that - spoiler alert! - that "Clark Kent" guy is in fact Superman.
This story became the standard against which all X-Men arcs are judged, and has been retold and revisited in almost every media. Sometimes, it's the only reason a creative team will even put Jean Grey in an X-Men story to begin with. It's been a good decade since the last screen adaptation to not treat it like the only story she was ever in.