Gosh, Jan, I don't think I'm really qualified to give you an answer, but I would think all those are required to be really successful. I do think though that without your first element, writing skill, you are probably a dead duck before you start! All of them are important though, that is why it is such a fascinating and often frustrating business!
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
I thnk that a key to successful writing is that writers gear their articles to a specific group of readers. Most of the publishers I have written for really emphasize this point. Writers should identify who their readers will be and ask: How do I grab the readers' interest so they will read my work? What do readers want to know about this topic? What essential information do I need to share with them? When writers do their thing just for their own satisfaction, articles often lack focus and miss important information. This can really bore readers or turn them off.