This sounds like a leading question, in a forum where a significant portion of the participants just like having fun, and another significant portion don't like having boxes forced on them.

Of course, the biggest nontechnical factor that makes projects successful is project management skills. (Whether these are worth the paper the diploma is printed on probably depends on where you're getting them.) These apply whether you're constructing a hi-rise, or writing software, or organising a Christmas party.

But that's not nearly as interesting as some of the other answers you got because a majority of people here don't influence the management of their projects, so they want to answer from their own perspectives, not yours.