Edited for Syndication: A rather unusual case. When the show was on Netflix for a while (albeit solely under the Iron Man name; nothing in Netflix's listings indicated that the FF show was included), the show was, for the first time since its' original broadcast, presented with its' original Marvel Action Hour branding and Stan Lee intros! (The only thing changed in this case was removed the outdated Marvel and Genesis and/or Saban logos in favor of the current Marvel Animation logo.) However, season 2 was rather strange since the season 1 Marvel Action Hour intros were edited together with the season 2 intros, apparently to remove any trace of the Marvel Action Universe branding they used for that season (possibly because for said season, IM and FF were joined by another New World show, Biker Mice from Mars, which Disney also owns but not the characters and underlying IP); it was very obvious as the voiceover suddenly switched from Jim Cummings to Tom Kane.
Recycled Script: Both seasons end with Doctor Doom claiming the power of the Silver Surfer. The second season production team actually did this on purpose. They felt that the first season's take on the classic comic story was poorly handled and deserved a better adaptation.
What Could Have Been: Plans for a third season included Sue's pregnancy (with She-Hulk or Medusa filling up the vacancy) and the return of the Sub-Mariner and Puppet Master.
Name's the Same: Nathanial Richards shares a name with Kang the Conqueror. They're different characters, though it's strongly implied the man who would become Kang was not only his direct descendant, but was inspired by the time-traveling ventures of his namesake who created the very utopia he grew disenchanted with, making him something of a Legacy Character.
Screwed by the Network: The reason the comic series came to an end in 2015? Blame Ike Perlmutter. Due to him being very bitter over Fox owning the film rights, he canned the series so that the 2015 movie wouldn't get any publicity from Marvel. Fortunately, after Disney bought Fox, the Four were revived in a new comic series in 2018.
Johnny's powers were limited, meaning at any point his flame could shut off.
During Chris Claremont's run on the title, he had initially planned for Kitty Pryde to be a recurring cast member, serving as a live-in nanny for Franklin Richards (this took place shortly after Excalibur was cancelled). Then-editor of the X-Men franchise Bob Harras wanted Kitty back in the main X-Men team.
When the book returned in 1998, Scott Lobdell talked of his big plans for it. He intended to turn C-list baddie Doctor Demonicus into a huge threat and have the Wizard engage in a dark battle of wits with Reed while leading a "Frightful Four that lived up to their name." He also had an idea of the FF discovering the cosmic rays had left long-term new effects. However, just three issues into his run, Lobdell left the book after battling editors and all his plans were scuttled.