Yeah it's a pretty hard one to answer given many people have had success using certain tactics. For some, social media promotion works really well, others create a lot of backlinks, and some don't promote much socially and still do well.
As Vickiw said, for returning traffic and credibility, the main starting point has to be quality of content as well as continuation of content creation.
On a slightly different note, each person would view 'success' as something different I suppose. For me, just finding HubPages was a success. Finding an outlet for my writing with an easy to use interface. When I made my first few dollars, that was also a success as well as seeing a couple of hubs get to 1st page of Google and go viral. Increasing my knowledge of social media tools I was unaware of before HubPages was a personal success too. Each person measures their success in different ways
What an interesting question!:-)