If I understand your question, I think you're asking what ideas and things that exist now could be erased from existence, and have considerable consequences.

I think antibiotics, pasteurization, printing press(biggest one) , anesthesia, poker [img]/images/graemlins/laugh.gif[/img], air conditioning [img]/images/graemlins/mad.gif[/img]

just too many to list.
Shooby