TubeMogul published an excellent guide to Web Video Marketing which they summarize into a "secret formula"...
Success = .50C + .15M + .20T + .15P
One example is my "Puppy Photo Everyday" video published in August. Below is the TubeMogul viewership chart showing how it has done since then.
.50C or 50% Content - The content was solidly in the "kitch" category being a pet video.
.15M or 15% Metadata - I'm not sure I did a good job on this. I could use some advice.
.20T or 20% Thumbnail - YouTube doesn't give you much choice. It's not always possible to get a good frame in the exact middle of a video, but in this case I lucked out and had a cute thumbnail.
.15P or 15% Promotion - I'm lucky to have a couple of thousand YouTube subscribers - giving me a jumpstart. But the big bonus on this video was getting featured in the Pets & Animals category. This put the thumbnail in front of everyone who choose to look at the Pets page on YouTube. As you can see from the chart, the video had large numbers for the few days it was featured.
Is it viral now? It is still getting play with about 500 views per day on YouTube.