Everything anywhere on this discussion board is fully visible to anyone on the internet*
It doesn't matter whether you post in one of the private forums or whether you hide the contents of your posts with BB codes. Since anyone can register on this forum, anyone can see your posts.
Even if you change your username, it doesn't matter – it isn't hard to find who the actual user is and what they changed it to.
Nothing on the internet is private. Once you post it, it's there.
It seems funny to me that people are surprised or upset that someone shows their posts or usernames in a YouTube video. That's not how it works, people. Again, once you post anything on an internet forum, it's out there on the internet. Whatever you post, keep this in mind. Having said that, I agree that not blurring out usernames in a YouTube video is poor etiquette (arguable).
All this sounds very scary, but you don't need to be extremely paranoid, unless you have a reason to be. Just use common sense and don't post specific personal details that can be used to identify you.
It's about cost/benefit and who's out there to get you. If someone wants to get you, and they have the time, resources, and money, they will get you. If you just want to hide from creeps, friends, and relatives, then just don't post stuff that gives away who you are.