Is you site or sites centered around things you are really interested in or just a way to make money? If you have both types do you find it easier to provide content for one versus the other and which is most successful? For me I truly enjoy being a father so I thought for my first stab at I website I would use that as my focus.
I would say some of my niches are a passion, some are just an interest. But it would be really hard to work on something you are not interested in. I tried to make a website about credit cards and simply had to hire writers for that. Now I am thinking about making make money online blog, because this is my pashion.
Passion. People who make crappy sites just to slap ads on them always fail. If you don't care enough about what you're making a site about, nobody else will.
My site is about making money. This is a interest of mine, but I've been looking to other alternatives/topics
An interest and reference for others first, an income source second. I doubt people who have no interest in their topic are going to be able to keep a site going for several years.
My most successful site isn't in a niche I love but interested in. I am passionate about websites though (since 2001) so I guess that counts.