Loss of hit points is the most common cause of death overall in our games. Usually this occurs because of unanticipated factors, such as a monster being tougher than expected, criticals or a PC getting cut off and surrounded by baddies.
Critical hits and the deadly pairing of hp damage with Con damage (i.e. max hp collapse due to Con loss and suddenly those relatively minor wounds become fatal) have claimed more PCs than anything else.
Most of our games have been low to mid level. At higher levels, I expect save or die effects to play a greater role in player mortality.