Jason shaves Josh's moustache before possessing him. No explanation is given.
According to the two-disk DVD set His Name Was Jason, which covers the making of the series over thirty years, it was an attempt to insert a homoerotic element in a series that had a great double standard in its sexual exploitation.
The appearance of The Necronomicon from Evil Dead.
Perhaps the Necronomicon's appearance was a bit of foreshadowing?
Special Effects Failure: After Jason gets stabbed with the dagger, agents of Hell sprout from the ground to drag him down under. The look of the hands...no. Reminds one of the giant foam fingers they sell at the sports games...
Squick: Jason tries to possess the body of his baby grand-niece. When that fails, he possesses his dead half-sister instead. Through her private parts.
They Wasted a Perfectly Good Character: Diana is the half-sister of Jason, who is hinted to know about his origins. But her role is just to die early in the film and have her corpse be conveniently present for the climax.