The murder weapon in cozies is usually a blunt instrument, i.e., a lapis lazuli paperweight, a fireplace poker, or Larry King. Poison is allowable as the agent of death in cozies but only if death is instantaneous. The ban on vomiting, I think, is in deference to the cat. The language of cozies does not permit the use of four-letter words. You can leave a cozy open on your kitchen table without fear that your ten-year old will adopt linguistic behaviors that will embarrass you before your bridge club.
Cozies usually take place in country houses or small towns.
According to the U.S. Census Bureau, people are far more likely to hate each other if confined in a small area together. The same is true of rats. You can read a cozy in front of your mother. However, you can read anything in front of a cat.