separate interests make someone separate rather than dating one's self. Still, some overlap is always good, especially if it's something that encompasses a fair amount of one's life. If two people watch a lot of TV, but have diametrically opposed tastes, then it probably won't end well.
Common interests are very important in a relationship, but it's also very important to keep your own identity and sense of self. A few is good enough- having none can make things difficult. You need to have things to do that make you happy on your own, so finding balance is key.