‘Don’t forget that the last sentence in your essay is the one that the examiner reads before he awards you a mark!’ – the warning once given by a canny English teacher.
An earlier ninevoices post commented on the advice given to writers about how first lines need to grab a reader’s attention. In short stories the last line is probably equally important – and difficult to write. There is no time for a gradual wind up of the story, the ending must come suddenly, like a ruthless slice of the knife. Ideas for openings and closings with their essential element of shock, surprise and fulfilment – these are often the initial inspiration for an imagined short story. But it’s interesting that some writers say they delete sentences from their original beginnings and endings to achieve a slicing effect which resonates and haunts the reader’s mind.
ninevoices 
I guess maximum impact in prose where every word counts and is a perfect choice is even more important than ever – research is apparently showing human brains are becoming more like those of a goldfish!