I follow the basic rule of never saying anything on my blog that I wouldn't want my husband or mother to read. You never know who is lurking and reading your blog. You may think your daughter's teacher doesn't know you from Adam, but she could very well be following your blog silently. Calling her inept after she gives out a bad grade is not going to go over well.
Keep secrets secret. A blog isn't the place for gossip. Even if you later delete the post, it is still likely archived somewhere and gossiping will one day come back to bite you. Even innocent secrets, like sharing gifts you are making or buying, can leak to the recipient and ruin the surprise. Blog about these achievements only after you have given the gift!
Finally, respect the privacy of others. It can be tempting to post picture of a new nephew or of your dad's high school picture. Keep in mind some people don't want their life all over the interwebs. Ask before you post personal things or images about other people. This saves you hurt feelings and awkwardness down the road.