I'd say the difference between seeing "wtf" and "f***" (with censors) is this:
"F***" is something that's censored. If it is already censored, at least partially, then its safe to assume it's innapropriate. With "wtf" and "stfu" you're assuming its innapropriate only because it can be. I'm assuming you'd argue that a person who types "f***" really and truley could mean "frig" behind those astricks, but I think you know that that's a little riduculous to say.
I'm not sure if I got my point across well, so I hope you understand my point...