The fact is that people here don't know you (unless, of course, they actually do and just happen to also blog here). I know that people here have no clue what I'm really like, because you can't see me and you can't hear the tone of my voice. I'm amazingly sarcastic (a good half of what I say is sarcastic) but I don't write much sarcasm because people take it seriously.
People on here also say things to each other that they would never say to somebody in person, because...well...they can. What's the consequences for doing so? In person, you might get fired, lose a friend, or receive bodily injury. On here, the worse that can happen is that you get banned.
The other problem is personal information. How much do you really want to share? It's a catch 22, right? LW has proven that sharing too much will come back to haunt you in the form of ammo for enemies, so how do you give enough info so that people actually know you but not enough to harm you?