Market Size
If the market you're addressing doesn't have the potential for a $500M opportunity a few years down the line, then you'll have trouble getting a VC interested today. (It's a simple problem of return-on-investment.)
It doesn't have to be that big today. In fact, it's better if it's not, because a market that big will attract bigger players. You want to be targeting an emerging market that will grow rapidly.
As mentioned earlier, there are at least two most-common paths to an emerging, high- growth opportunity
- A new way to solve a problem, where the problem has existed for a long time, but the way to solve the problem is new (example: the iPod was a new, much improved way to carry your tunes with you; it was made possible by micro-sized hard disk technology)
- A new problem that didn't exist before (example from the the book "The Innovator's Dilemma": before there were laptops, there wasn't a demand for 3.5 inch hard disk drives)
So what is driving the high-growth in the opportunity that your company addresses? You need a compelling argument that you've got a high-growth opportunity.
Market Research
The problem with emerging markets is that there's no reliable market data for those markets.
My favorite emerging-market story: there was a car designer at Ford who came up with a design for a new kind of car that he thought would be very practical for baby-boomer families (which suggests the "change" that was driving the opportunity). But since this was a new type of automobile for which there were no market studies showing the market size, Ford decided to pass. So the designer took the idea to Chrystler, where the CEO liked it. And so the mini-van, a very successful product-line for Chrystler, was born.
So if your emerging market is so new that there are no market studies to point to, then you'll have to do what the designer from Ford did: argue the compelling nature of the change that will create the opportunity. (And take comfort in the fact that the best market opportunities for startups are those for which there is no market study from one of the leading market research companies.)
On the other hand, if you're addressing a market that market studies say is a billion-dollar- plus opportunity today, then you'd better be ready to argue how it is that you'll beat the big guys that should be addressing that market already.