I think passion is very important. Overall, the better your understanding for the application/use of the work you're doing, the better your overall chances for success. Having passion for the application and use of the product will allow you to think about how it will be used, what the users will value the most, etc.
A recent startup I worked at was trying to create a product that was used for audit compliance (SOX, PCI, etc.). The problem is that they really didn't have anyone truly passionate or informed on the subject. The first several versions of the product greatly reflected this.