Dean Dad has a good post up about "boundaries" and blogging--where does one draw the professional line? My own rules have always gone as follows:
1. No references (other than positive ones) to colleagues, administrators, etc. If I've got a problem with someone (which I almost never do--it takes a lot to get me riled...), then I need to take it up with said someone in person, not on a blog.
2. All references to students must be generalized as far as possible. If I complain about an error or oddity, it's because a lot of students committed the same error or generated the same oddity. (Even the Titus Andronicus outburst mentioned below involved more than one person.)
3. No discussion of confidential matters. Individual plagiarism cases, problems with graduate students, etc., are all off the table. So are specifics of job searches, departmental bureaucracy, and the like.