I learned hard way creating websites.
Once it dawned on me that we should have social networking on our sites, then another day that a mobile version is required.
That we should register with Google Analytics and Google Webmaster Tools. Analysis with AWstats is also crucial for visitor stats. Successful sites are most likely on VPS or higher hosting.
Choosing right CMS is also important. Implied is good original content.
Without making further discoveries .. I'd like to know from pro-webmasters the common traits of all successful sites.
UPDATE: I'm not asking about the content or the quality of it. I want to know common features/functions of successful sites.