Like in the comic books, will Gwen Stacy become Peter's first love and die at the hands of the Green Goblin (or rather Spider-Man's webbing)?
Due to the show beinng cancelled prematurely we will never know. However, Weisman expressed concern in making audience fall in love with Gwen Stacy the character and not think of her death as her defining point.
Does Cletus Kasady make an appearance?
Yes, but he makes a cameo appearance in The Spectacular Spider-Man episode Reinforcement. He is shown as a patient wearing a Santa Claus hat in Ravencroft, during a group therapy session with Doctor Octopus and Electro. Kraven the Hunter attacks to break out Electro and Doctor Octopus.http://i288.photobucket.com/albums/ll198/evilash1990/SpectacularCletus.png
Who is the Green Goblin?
!! Warning major spoiler if you have not seen episode 13 of Season 2!!
(Hover your mouse over it to show the message)In episode 13, Final Curtain, it is revealed that Norman Osborn is the Green Goblin. Although Spider-Man suspected him at first, he cleverly escaped this suspicion by staging his son Harry as the Green Goblin. The times that Norman and the Green Goblin were seen together, one of them was the Chameleon in disguise.