How were the Fantastic Four different from the Justice League?

The Fantastic Four were created by Stan Lee and Jack Kirby in 1961 specifically to compete with the Justice League, but they were designed to be a stark departure from the established team dynamic.1

The primary differences between the two groups include:

• Identity and Secrecy: While Justice League members typically concealed their true identities behind masks and superguises, the members of the Fantastic Four saw no reason to hide their identities or their powers.1
• Interpersonal Dynamics: The Justice League was defined by camaraderie and teamwork. In contrast, the Fantastic Four were portrayed as a dysfunctional family whose members bickered incessantly and dealt with internal personality conflicts, even while setting those differences aside during crises.21
• Setting: The Justice League operated out of fictional cities like Metropolis or Gotham City, whereas the Fantastic Four resided in the real-world city of New York.1

Ultimately, while the Fantastic Four were inspired by the existence of the Justice League, they were intentionally crafted to be "the JLA through a refractive lens," prioritizing realistic human flaws over traditional heroic archetypes.1