Do what you are good at!
There is nothing wrong with trying to expand as a freelance writer. After all, this is one of the best ways to make more money, and hopefully break into new markets. But with that being said, you should do what you are good at a lot of the time. This is not a new concept, or one that you should have a difficult time coming to grips with. When you work with subjects and projects in which you excel, you will find that success comes much easier.
For me, this means sticking to certain topics as well as certain types of projects. Although I am open to learning new material and taking on unique jobs, I try to stick with those with which I am most comfortable. As far as project types are concerned, I like to work on sales letters, keyword and feature articles, and marketing material. Does this mean that I turn down other work? Of course not. In fact, I am ghostwriting a book at this time and am working on a long research project of my own. But for the most part, I like to spend the majority of my day on projects that I know I can handle in an enjoyable way.
But how do I decide what I am good at? This is a question that is asked of me time after time. The only way to answer this question is to take on a wide variety of projects, and then reflect back on them when complete. When I first started working as a freelance writer, I though that I would only want to write feature articles and books. Of course, this changed a couple of months into my career and is still morphing as each day goes by. As you complete more projects you will get a better feel for what you like, and what you would rather avoid.
When you do what you are good at, you will find that you are having a lot of success and that you are happy with your progress as a freelance writer. Just remember: this does not mean that you should be afraid to think outside the box and take on new projects.