Mainly luck. A little bit of awesomeness but mainly luck; if you have no traffic in the first place no one will really see your site. It's kind of like youtube; there are millions there but nobody get's seen by the millions.
Usually if its a quality website that a certain group of people like, they will naturally share the website. The word of mouth is still very strong. Look at minecraft.
it has to appeal a crowd and if its content is good and so is the style of the website and simplicity. word will get out and there let nature take its course.