While questions are supposed to be limited to one question per post, I left the question as it was asked without editing it down. For me, all of the questions seem to ask the same thing--just different ways. Essentially, each of the questions want one to examine the elements of Gothic literature which make it both terrifying and "fun" (in regards to terror).
With any reference to Gothic literature and terror, the first text which comes to mind is Mary Shelley's Frankenstein. Mary Shelley defines the creature's physical attributes in a very limited nature. Outside of his height (eight feet tall), black lips, dull yellow eye and skin, and "pearly white teeth." beyond these very specific characteristics, the creature is not described. Shelley does this in order to allow the reader to create his or her own mental picture of the creature. Sadly, the picture many readers possess are linked to the common representations made by different medias (green skin, bolts in his neck, huge feet, and walks with arms extended). I bring this up because Shelley's lack of a detailed description allows readers to create the most horrific creature they can think of.
For me, the most frightening thing (inspired by Gothic Literature) is the crumbling setting (decaying castles) which holds a frightening zombie-vampire-like being which is extremely intelligent. Like Shelley's visually horrific, yet insanely intelligent, creature, this being would not only be able to kill me, it would be able to outsmart me as well.