"Great market" /= "hot market"
Great market to my mind is a market that has a lot of users who will use your product (and better yet, you can make money off of them using your product). Maybe I will write a post on how to identify a great market, as that is a pretty big topic. But one example is often markets where user behavior is fundamentally shifting, or where there is a sudden change (e.g. adoption of truly smart phones) leading to opportunity.
I think certain areas in ecommerce are a good example today of changes in user behavior and business model leading to high growth companies (Gilt Group was just the beginning, and some really cool companies are getting started today around fashion/women's goods online).