Based on my own experience teaching people who knew absolutely nothing about programming, the biggest factor is visibility of results, i.e. making programs that

Based on my own experience teaching people who knew absolutely nothing about programming, the biggest factor is visibility of results, i.e. making programs that do stuff you can see and touch (games are the best). Certainly, the immediacy and speed of iteration of Python and JavaScript trump that of C these days. My choice today is actually JavaScript, partly because of ubiquity, partly because it's very easy to do something interactive and visually interesting (Canvas), and partly because its simplicity soon leads into its shortcomings. Python as a language is "batteries included", and I believe in the value of students bumping into barriers and limitations so they can properly value the reasoning behind solutions. In that sense, the value of C is in teaching students about lifetimes and ownership of resources.