(or "The Modern Hero")
Moon Knight isn't exactly what you'd call a "big name" in the superhero comics business. Oh, he's had a respectable amount of success; a few series, but none that have managed to pass the sixty-issue mark, a cult following that keeps him from becoming cannon fodder for the next big crossover to come along, but when the average comics fan thinks of Moon Knight, they probably relegate him to the category of "Batman rip-off".
Which is brutally unfair on a number of levels. For one thing, it suggests that "Batman" is the template for any masked hero without actual super-powers. Zorro, Doc Savage, the Spider, and the Scarlet Pimpernel all might have something to say about that. For another thing, Moon Knight is a far different animal from Batman, with a storytelling engine all his own; indeed, you can point to Moon Knight's creation in the Bronze Age as a template for a whole different sort of hero, a thoroughly modern superhero that draws on the culture of the 70s and 80s in the same way that Batman drew on the pulp heroes of the 20s and 30s for inspiration.
You can see in Moon Knight's origin a reflection of the troubled times he was created in; he started his life as a callous mercenary, an ex-Marine who had turned to his talent for violence as his only marketable skill. (With the Vietnam war winding down and many soldiers returning to an unsympathetic or outright hostile homeland, the notion of the disillusioned vet was heavy in the public consciousness. Only a year after Moon Knight's first appearance, 'Taxi Driver's Travis Bickle would be portrayed as a vet.)
Of course, Moon Knight can't stay a callous gunman if he wants to be published in a Marvel comic, and after a change of heart brought about by a near-death experience and an Egyptian god, Marc Spector sets out to become a costumed vigilante just like any other. But Doug Moench, the character's creator, made him entirely unlike any other by making him more like the "vigilantes" we see in the streets than in the comics. Spector was a damaged individual, developing his alter-ego into an entirely separate personality--and in fact, developing many identities, each with a personality all their own. His wealth as a mercenary enabled him to live a playboy lifestyle as "Steven Grant", his need to obtain street-level information brought about cabbie "Jake Lockley" (note the significance of Lockley at just about the time 'Taxi Driver' was released. Perhaps not intentional, but the characters shared the same zeitgeist.)
Nowadays, we think of this as Batman's exclusive schtick. Everyone knows that he thinks of Bruce Wayne as the fake identity, and Batman as his "real self". But Moon Knight was doing it first, and doing it with more sincerity--to him, every identity was a "real self".
Also, the very notion of "vigilantism" was being re-examined. Instead of being seen as a hero who helped the law, Moon Knight and other, similar characters were seen as people who operated outside it. After all, when you boil it down, Marc/Steven/Jake/Moon Knight was just a guy who wandered the streets beating up people he thought were bad. This series was one of the first (alongside the Punisher) to take the position that this might not necessarily be an automatically good thing.
Moon Knight's antagonists, as well, were damaged individuals--Stained Glass Scarlet was an ex-nun who ruthlessly pursued her own criminal son, Black Spectre was another Vietnam vet with a grudge against society, and Morpheus was a crazed insomniac with psychic powers. All elements of the same damaged society that the media of the day examined (again, 'Taxi Driver' is almost a required reference here.)
So why, if Moon Knight has tapped into the modern age so well, does the character not seem to get the kind of following that 'Apocalypse Now' or 'Raging Bull' has? Part of it (much as it pains us to say it, given that this is a series of columns on the elements of a series that stay constant as creators come and go) is the departure of Bill Sienkiewicz as artist. Sienkiewicz's magnificent art reflected Moon Knight's unstable psyche, growing in sophistication as the series went on, and it's never really been the same without him.
Also, subsequent writers never managed to balance Moon Knight's alienated persona and his superheroic nature very well. Either he was written as just another street-smart vigilante, or the "damaged" side of his psyche was played up to the point where he was too vicious to be sympathetic. In a world where everyone imitated Moon Knight, he had to become more extreme to push the envelope, perhaps too much so.
And ultimately, that's the factor that harmed Moon Knight's popularity the most. Having seen that he was the future of superheroes, everyone jumped to position themselves in front of him. Batman's villains started to emphasize their fractured psyches over their gimmicky crimes, anti-heroes became more savage and brutal, and everyone suddenly gained a tortured soul and a borderline-psychotic personality disorder. And Moon Knight went from being unique, the Modern Superhero, to just one of the pack.