Spider-Man earns its PG-13 rating mainly for violence. When Spider-Man goes head-to-head with the Green Goblin, things can get pretty intense. People get shot. Bones get broken. Pumpkin bombs explode in people's faces. There's a fair amount of blood.
Speaking of heads, Green Goblin's mask might be frightening to younger Spidey fans, too—although it's nothing like the original design the producers toyed with using. That thing's pure nightmare fuel.