Originally, the Human Torch was going to be in the series with Spider-Man and Iceman. Licensing issues at the time prevented the use of the Human Torch and so Firestar was created to fill the open spot. Firestar has similar powers as Human Torch, but doesn't have the appearance of being on fire.
Only three villains made return appearances after their first appearance on the show: Magneto (in a brief flashback in "A Firestar is Born"), Electro (in a cameo appearance in "Attack of the Arachnoid"), and Videoman (who appeared in "Videoman", "The Origin of Iceman, and "The Education of a Superhero").
Doctor Octopus was the only super-villain to cross over on three shows (this one, The Incredible Hulk (1982) and Spider-Man (1981). Michael Bell voiced Doc Ock both this series and the Hulk cartoon which aired on NBC on Saturday mornings at the same time.
The character Videoman was created exclusively for this series. Initially, Videoman was a villain, but with the video game craze of the 1980's, Marvel thought the character had more potential as a hero instead of a villain and set him up to be included in the X-Men for their own series pilot.
Wolverine's appearance in this show has him speak in an Australian accent, voiced by Neil Ross, despite the character being from Canada in the comics. Wolverine also has an Australian accent and again voiced by Ross in Pryde of the X-Men (1989) which is also in continuity with this series.