What factors influence how much of an online video people will watch?
There could be any number of factors as to why people quit watching a video, but if you want to entice them to stick around, there are a few tricks.


Yes, "content is king," but CONTEXT is arguably queen— and let's be honest, who really rules the castle? That is to say— even if your content is great, if you aren't getting it to the right audience, it won't matter. Make sure the people who would WANT to see your video are finding it at the right time and place. Context.
Pay-off. A viewer is more likely to stick around if they know the pay-off at the end of the video is worth it. What will they learn? What mystery will be solved? What's the burning question? etc. Give them a clue in the beginning.
Challenge? As a twist to the "pay-off" bit (and especially if your end goal is social media shares and conversions)... Remember the whole KONY 2012 thing? They got millions of people to watch a 20-something minute documentary, and then spread it virally. Why? Beyond just compelling content: they challenged the viewer to finish it at the very beginning. The first lines were similar to "The following video is a social experiment..." I.E. "Let's see who makes it all the way through and then cares enough to do something about it." 


It was a challenge, and human nature led people to take it on.

So there's my 2 cents.