There are quite a few divergent questions here - and I'm not sure how I feel about some of the issues raised.
I believe there is a difference between the "can I write about my students in my blog" and "can I discuss other students/ staff with other students / staff""
For me, the first question depends. It depends on how you do it, the context in the classroom, how old the students are, what your purpose is, and so on, and on and on. I think there are many cases in which it would be perfectly OK to share a classroom experience on-line (or off) and, in fact, it can be a very good thing to do so - and not just for purposes of blowing off steam! :p
The other question - that of discussing a fellow colleague with students, or students with a fellow colleague - well, here I am probably biased.
I think it is highly unprofessional to discuss fellow students or colleagues with students - if the discussion is related to your professional relationship with them. Of course, it depends on the kind of relationship you have with your students, but in my experience, that kind of behaviours will erode trust. "if she says that about so and so, what does she say about me?"
Getting involved with a student's discontent with a colleague - also very dangerous. Most of the time the picture is, at best distorted. You are hearing half the story, and often filtered through a host of misunderstandings. Usually the school will have an person who is responsible for dealing with those issues, and you can seriously complicate matters by rushing in blindly.
I don't believe that teachers should band together against students, or that the teacher is always right. But from my experience, it is very easy to get manipulated into a situation where you are seriously undermining a colleagues authority.
And there we have it. Authority. Not everybody handles it in the same way. Also - there are different roles a teacher may fulfil. Leading a workshop with adult professionals is different from teaching day by day in a school. But in the end - there is a specific relationship between a teacher and a student. While it does not exclude friendly behaviour (like being friends of facebook) it does exclude being friends (like sharing your emotional burdens).
I've been in situations where students attend a staff meeting. Nothing wrong with that. But then we start discussing another students issues. Isnt that a betrayal of trust?