What makes a YouTube video go viral?

Humor.
At least that's what works for me.
Make me laugh, I'll share it.

I think it also depends on the region and the target audience.
I don't believe that there is a formula; it is a combination of being at the right place at the right time, smart SEO, clout, and originality.

Sharing on social media, choosing an influential person to “seed” a video is essential.
Make this video easy to find.
Ask yourself what would you type into Google to find this and then ensure these tags are used.

Before I answer this I really have to share this - it really irks me when I come clients say they want a viral video.
Doesn't virality depend on the audiences reaction? And no matter what specialist say there is no 100% foolproof formula that can guarantee that the audience will react in a certain manner.
Thus there is no 100% foolproof formula to guarantee virality.
Having said that yes, humor is one of the elements most frequently used and audience connects with it.
Videos that are provocative in thought also get virality but the negative kind.
Originality of thought and executing that thought beautifully is also important.

A video with a conceptual message that can grab a viewer's attention in seconds...

Keywords, content, connections and social media platforms.
- Keywords/tags are essential to find the video in search results.
- Content is what makes it or breaks it: either you find it worth it or not (and worth it does not necessarily mean you like it, you can absolutely hate it and still want to share it) - Connections: Are those you want to share this video with, the more connections you have the better for the video to go viral.
They in turn will share it with their connections and so on so forth..
- Social Media Platforms: By far the fastest and most efficient means for a video to go viral.

A few days after I asked this question, I read a very interesting article on Inc.com (here: http://www.inc.com/john-brandon/new-rules-for-viral-ads.html) where Andrew Lipman, a manager at Audi, told the editor that the success recipe for any video to go viral is for that video to create an event, i.e something that people start talking about and sharing with each other.
When I read that, I immediately remembered Psy's "Gangnam Style".
The moment I saw that video I felt a huge urge to share it with everyone I know: siblings, friends, coworkers, parents (yes, parents - my mom fell on the floor laughing!).
This is exactly what a viral video should be; a sharable video.
Similar to any viral content on the internet: content that people would share.

If I knew that, I'd be richer than Bill Gates.
All I got right now is that it has to be word-of-mouth-worthy.
Of all the viral videos that I've come across, I heard it from someone (who heard it from someone, who heard it...
you get the idea).
I didn't search for it.
It found me.
And I didn't need to share it, someone else overheard it from me or the person I heard it from (and the contagion continues).
It has to be something new (one of those why-didn't-I-think-of-that ideas), or refreshing (meaning old idea but new way expressing), and something that sets off fireworks in each of our primal brains.
Content-wise, sometimes it will feel like the universe has aligned and the Truth is beamed straight into the very fiber of your being, everything makes sense, everything is as it should be.
Other times it makes no sense at all like cats singing or gorillas playing a Phil Collins song on the drums.
That's all I got.
I guess Bill Gates shouldn't feel threatened yet.

Depends on the "WOW" factor.
If you can make your viewer go WOW, you definitely have a winner and people will start sharing.
It depends on how you can connect with the viewer be it humor, emotions or even surprise at seeing something completely new.

1- Video quality,

2- Uniqueness

 and...

3- Prompt upload (if its a hot news, drama, movie or program then if you publish it quickly before more people upload this then your video will get remarkable increase in views)

Youtube is viral because it is multicasting without any downtime and then comes compression factor. As we know youtube have it's own compression technique i.e. VP8 now what  here is done that broadcasting is in segment level so even live streaming is possible. Now they are using adaptive streaming so according to bandwidth video is streaming & depends upon quality. Anybody who knows model of communication which are as follows.

message>sender//encoder>channel>reciever/decoder>reciever. feedback is possible between sender and reciever and even noise is introduced that is cancelled out.

Now if you have any communication that is based on above principle and uses human sense organ fetures like eyes for reading whic is video,  tongue for taste here like button, ear is to listen that is audio, communicate in group here like local issues. now if someone able to bring smell feature, skin feature in his communication then he will be more popular than youtube. com eon guys! Man is social animal