With "The Pit and the Pendulum," Poe gives us a story as written by a man saved from the brink of death. Though our narrator has escaped execution, he cannot escape from his memory – and it shows. Every paragraph is filled not only with an intense description of harrowing events, but also with a serious investigation of their meaning. He's always pondering, or reflecting, or thinking. This philosophical thought process is in sharp contrast to the brutal death we're imagining is possible for him. Maybe that's the point?
There is also an urgency to the writing – we get the feeling that our narrator needs to tell his story, and fast; it's as if he hasn't gotten his head around the whole "I've been saved!" thing quite yet.
And finally, considering the horrible events narrated, it's no surprise that "The Pit and the Pendulum" is overwhelmingly dark and serious; as the narrator himself says, the only hope he experiences is that