Matthew, when I visit forums, registration puts me off posting. I want people to be able to contribute without hassle. But can appreciate that there are repercussions. Registration doesn't however stop people being dickheads - see YouTube and Facebook comments for enough evidence of that. Beyond that, the limitation comes from the time I have available plus legacy tech issues (ownership of posts made under an alias but not linked to a user account, etc.)
KingJames, yes, there would be a way to do something like that. It could be a hash of their IP address and only show if someone wasn't logged in. But needs a way to be unique enough but recognisable. Some sites use odd sequences of shapes and colours for this, but I don't know how effective they are - I usually ignore them because they can't be judged at a glance.