Thats a classic question for many wannabe bloggers. Now what should a blog really have in it. Your personal opinions on something, complaints, criticisms, your dreams. It could be anything which you are comfortable putting up, really easy. However, the thought of what others would think about it should have forced hundreds (may be thousands) of people to delete their well constructed posts. There is nothing to worry as long as you want to blog on something. The only important thing i believe is just what you want to do with your blog, and forget about the rest.
Yes a bit of caution is not pull yourself into troubles by touching sensitive issues or confidential matters.