Agree with Tom here that the viral loop is crucial (as well as "simply" having a great product). But a big problem is getting that initial traction! And then a bigger problem is reaching a critical mass.

So how do you start that snowball?
What worked for me in the past, and is also working for startups I advise to today - is the development of automated tools that will reach out to your target audience. The market/industry is irrelevant, as long as you can identify where online your target audience is active, and then build tools around these websites/platforms.

The best case I can give you is SweetIM, where I was the second guy (Growth Hacker and Product Manager). When I joined we had a product in development and 0 users. When I left we had over 100M users and millions in revenue (check out TechCrunch story about it: http://techcrunch.com/2012/11/08/holy-emoticons-batman-perion-network-acquires-sweetpacks-for-41m/)

I've developed automated zero-cost user acquisition tools that worked and generated massive amount of users. Today I'm using the same techniques in order to generate traffic and installs for my current startup, as well as other startups I advise to.

In general I can say that you need to think outside the box! Otherwise how would your app stand out from the millions of apps out there?
Not sure what stage you're at, but Ideally you will have someone in your team that has a solid background in Product Management, Marketing, Computer Science, and a deep understanding in user-behavior / cognitive science. A person that will find the best way to reach out to potential users, he/she will know how to form habits among users, what will make them keep using the product, and what will make them invite the rest of the world!