I went through my friends this morning and put a vast majority of them as acquaintances. When I post any comment I can choose if I want it to all my friends or just to friends but not acquaintances. This way I protect my privacy when I do post something that I don't want the world to see...
I also un-friended a few. But it was because I play games with them and they don't play anymore.
What happens when you're un-friended is that you don't see their posts and they don't see yours anymore. They don't have the ability to see your photos and so forth.
I also don't follow anyone I just play games with. I don't care to see their general posts, I really could not care less if their grand kids are adorable...this keeps my wall cleaner and less clutter.
If they block you then when you hunt for them you won't be able to find their wall anymore. It will be as if they don't exist. You and that person could both be commenting on a mutual friends post and neither of you would see the others words or names or anything.
I have several people I don't like in my community blocked and I often see a comment in a thread where I figure out that person has commented because someone will say something like "Wow, that was silly John. You really caught the football behind your back?" and I can figure out that's a story I've heard before so I know that person has told that story but I can't see it at all.
So, if they've unfriended you then you should be able to find them on FB and ask them. It's not unheard of for FB to have a glitch and some of your friends disappear. I've had friends send me a message, it goes in the other box, and they say Hey, we're not friends any more, did I do something wrong? and I say no, it must be a FB issue.