Peter Parker transformed comics because he was the first great everyman superhero.
Amazing Spider-Man #1 hit shelves 50 years ago, on March 10, 1963. Since then, Spider-Man has spawned four—soon to be five—big-budget movies, nine TV shows, a stage play, a radio drama partially masterminded by Brian Mays of Queen, a few dozen video games, and, of course, thousands of comic books and toys. He's a major figure, and he deserves to be: Spider-Man redefined our idea of a hero by making superheroes a lot more relatable than they were before.
To understand how revolutionary Spider-Man was, it helps to understand the most important hero who came before him: Superman. Created by Jerry Siegel and Joe Shuster in 1932, Superman debuted in Action Comics #1 in 1938 as more force of nature than fully fleshed-out character. Rather than fighting the colorful super-villains that would later define him, Superman attacked a wife beater and rescued a woman from being wrongfully executed by the government by storming a governor's mansion with proof of innocence. The creation of Superman led to plenty of direct imitations—Captain Marvel being the most popular off-brand Superman, I believe—and eventually the complete dominance of superhero comics over most other genres in comics, a status quo that survives to today.
In 1962, Steve Ditko and Stan Lee created Peter Parker, better known as Spider-Man. Spider-Man was something different. Instead of growing out of a pulp tradition like Superman, Spider-Man's forefathers were superheroes. He was a twist on a 20-year-old formula, but managed to become a paradigm shift at least as important as Superman.
Both Superman and Spider-Man are heroes, but the differences between them are profound. Where Superman was a grown man, Peter Parker was a bullied teenager. Where Superman enjoyed the adoration and trust of those he protected, Spider-Man was regularly vilified in the press. Where Superman was motivated by his innate goodness, Spider-Man had to work to be a hero, and often fell short of the mark.
Both characters have tragedy as part of their origin. Superman's home planet Krypton exploded when he was a baby, and his birth parents placed him in a rocket and aimed him at Earth. Superman's tragedy is set at a distance from his day-to-day life, but Spider-Man's is much more personal. After gaining powers from being bitten by a radioactive spider—a reflection of the atomic paranoia of the time on par with Godzilla—Peter Parker chose to become a wrestler and try to make some money with his powers. Instead of doing good deeds with his powers, he made a selfish decision that led directly to his Uncle Ben's death, leaving his Aunt May widowed and adrift.
The shift in approach there should not be ignored. Instead of being a self-assured man with functionally infinite resources for fighting crime, Spider-Man is a lonely and bitter teenager who struggles to help his aunt keep the lights on. He is indirectly responsible for the death of a loved one, and that tragedy provides the fuel for his crusade. Instead of fighting crime as a show of his own innate goodness, Spider-Man begins to fight crime as a way to atone for his own sins.
Spider-Man still struggles with his day-to-day life like the rest of us, and that fact serves to pull superheroes down to Earth. Parker is thrust into the role of man of the house after his uncle's murder, and he struggles under the weight of that burden. He works hard for next to no credit, often ends up rejecting profit in favor of doing right and honoring his uncle, and puts up with a hostile working environment to make ends meet.
He's still a teenager, however, and that counts for a lot. Underneath his mask, Parker is smart, but that smartness comes alongside the kind of bitterness that festers when someone feels underappreciated. He constantly thinks about showing up Flash Thompson, his primary bully, by humiliating or attacking him. This is less than heroic behavior by any definition, but you understand where he's coming from even as he's behaving poorly. He's a boy that's in over his head.
When he pulls the mask on, however, Parker becomes The Amazing Spider-Man. "Becomes" is the most important word in that sentence. He doesn't have the innate goodness of Superman, but he does have some idea of how superheroes are supposed to act. It's easy to miss, but the Spider-Man persona is an act. He's a teenage boy's swashbuckling, one-liner-tossing idea of what a hero is supposed to be like. He is suave, he is fearless, he is daring, he is cool, he is everything that Peter Parker wishes he was.
That performance element is vital to understanding Spider-Man, and an inversion of the relationship between Clark Kent and Superman. Superman plays Clark Kent as square and uncool as he can possibly be in an attempt to secure his secret identity, but Kent and Superman have the same righteous nature at their core. (Kent is often portrayed as an investigative journalist, after all.) Peter Parker, on the other hand, is a teenager first and a superhero last. He has to work for his heroism. He has to remind himself why he fights. When his faith cracks and he quits, he returns out of guilt. He eventually grows into his own righteous nature and to embody the phrase "with great power comes great responsibility," but it takes some time.
Spider-Man has failure at his roots, but he isn't defined by his tragedy. He's defined by the fact that he keeps struggling and striving to be the person his uncle knew he could one day become. Everyone knows what it feels like to have someone else's expectations sitting on their shoulders, and that's part of why Spider-Man works so well. Often times, Parker's personal drama seemed more dangerous and exciting than the cadre of villains who would do him harm. It's easy to figure out that Spider-Man isn't going to die, but disappointing his aunt, missing a date, or losing his job are all within the realm of possibility.