There has been a lot of discussion about being passionate for your niche. But, the first question I would ask is, "What is your intent for the niche?" If you want to build a site where you will continuously be adding content, then it is important you have a passion for your niche. Otherwise, you'll find yourself doing other things. If, however, you want to write an ebook that solves a specific problem, then your enduring attention is not necessary (other than keeping people coming to your site). You can even farm the writing of the book out. In either case, you want to look for a topic that has a relatively high number of searches compared to the number of sites. The sites should have relative low PR values. And, you would like few competing PPC ads. This way you can get pretty good positions in search results for your keywords and can have inexpensive PPC campaigns.