firstly,the event must be novel.as a joke goes "a dog bites a man is not a news.it will only be a news if the reverse happens".secondly,it should have large impact.the more people or countries are influenced,or the more attention is paid to the event,the more newsworthy is the occurrance.finally,it should be unexpected.a bomb attack is not news in Iraq becomes people kind of get used to it (sadly).however,if it happens somewhere rendered very safe,such as singapore,it will become a good piece of news then