Way back when AWS EC2 was announced by Jeff Bezos. He showed a graph where a startup needed to scale fast because startup's launch went viral and they were able to add more power (machines, cpu etc) quick. OK, nice. But then the first launch hype was over and EC2 allowed them to scale down equally fast to safe money. That was the killer feature for me: servers rented by the hour.
Reminds me of this Jeff Bezos interview from 2006 where he talks about S3 and goes, "It is hard to come up with a web application that doesn't need to remember things".
