I am constantly splitting my brains about how I can work on an "important" problem; something where my expertise will be relevant years after grad school, too. And I don't want to become a professor and there is a chance I might choose to go back to my home country (not much of compsci products, but very good e-commerce and banking); so my expertise should be of value even in a consumer-of-technology (not producer) economy.
If anybody has ideas about how to pick an area for research, I'd love to hear.