Several people have posted about how funny they found ch.1 of CoO, and that they simply couldn't take it seriously. I laughed too, but I wonder if the author intended his novel to be such an obvious joke when it first appeared. Wasn't CoO one of the *first* English-language gothic tales? As such, I think many of the classic elements that we jaded 21st-century types are so used to (portraits that come alive, trap doors that may lead to danger or freedom, etc., etc.) would have been new and original to their first audience.
Of course, Shakespeare had ghosts and maidens in distress, but I have a feeling that many of the elements that we take for granted in CoO would have been quite novel and unheard-of to Horace Walpole's first readers. The fact that they are so common now simply shows how successful the concept was. I compare the people who had nightmares after reading this novel to someone who has just seen their first horror movie. If you've had a steady diet of them since you were 13, you naturally notice how cheesy and repetetive the special effects are, but if this is your first time, you're just scared stiff!
BTW, this is the first time I've been asked to take the phrase "frothing at the mouth" seriously (at least I *think* I'm being asked to take it seriously ;-). Usually this phrase is used ironically, but I think Walpole actually expected his readers to react with a shiver at the thought of someone being so frightened that they were frothing at the mouth!