Things usually go viral because they generate a strong reaction that provokes social sharing.
Often that is some sort of novelty/awe, laughter or anger/urgency.
People share things that amaze them or that are unique, beautiful or otherwise exceptional in quality. Personally I am a visual artist first & foremost so I try to draw attention with outstanding graphics and artwork, as I get the sense that's my best shot at standing out here.
But if you make people genuinely laugh with something hilarious that can work too.
And a lot of times when people are shocked or angry about something that is happening or about to happen, they'll tell others and mobilize against it, even when the thing that upset them in the first place turns out to be a total fabrication (i.e. phony clickbait political news stories spreading on Facebook)
Some other thoughts:
-- incentives. If there's an upside to sharing, then people will more likely share. (I.e. a discount for those who post about a game on social media).
-- time limits. People might buy, and spread the word, if a really good sale is available, and is not going to last long. I'm going with a discounted early access [just one dollar] and will keep the pricing as low as I can justify even after my game's fully finished, because I am hoping it'll provoke impulse buys and that the larger volume of sales will compensate for the small amount earned per sale.
-- low barriers to entry. That is, if the game's playable for very cheap or better yet, free it will be easier to access and more likely to spread. This is why 'freemium' and ad-supported games have become a common thing; they can be downloaded without spending any money. Of course, such game experiences are generally pretty flawed and low quality, but they tend to spread anyway. A limited but still fun and playable freeware game demo, though, can also potentially be an effective way to offer a tantalizing freebie without compromising the full game with broken freemium game mechanics. I am likely to release some small portion of my project 'Miniature Multiverse' for free at some point, in keeping with this concept.
Virality is not very predictable. Orchestrated, costly promotional campaigns, however, are [relatively] predictable.
You can do a lot right and still not get much attention simply because the indie game scene is so crowded... and big studios will likely drown your work out. Your odds are actually better if there's some traditional promotion in the mix to get the ball rolling. Posting on relevant gaming forums helps, and leveraging all your various social networks. You can also identify people you follow on social platforms who are popular and message them with information about your game. You can even give them a free copy of the game, or an exclusive bit of media [in advance of the game's release] that nobody's seen before, that they can post... it benefits them because they feel and look like they have an 'inside scoop' on your project, and you may benefit from the exposure. Paid advertising can also work but only if it's lean enough [well targeted, low cost per acquired customer] to be effective in generating more revenue than it costs.
I HATE when people ask if I'm going to make the next Angry Birds. I do not have a million dollar online ad campaign backing my launch like Rovio did. People think that game 'went viral' and to some degree it did. But really it became a phenomenon mainly due to a strong promotional campaign right out of the gate and good timing early on in the life of iOS when fewer than 100k apps existed on the platform. It was not indie, and it demonstrates how effectively big ad campaigns at launch can propel a game into a widespread hit. It also demonstrates how if you can force your way into some top 10 or top 20 list, like 'most downloaded'/'most popular'/'best selling' you can multiply the exposure generated by your ad campaign and make vast sums regardless of your game's real merit, if you have a studio with enough cash to make that happen.
That's about as much as I can think of about virality at the moment. If anyone else has suggestions I missed, feel free to add them to this thread.