[Kate!]

It's an interesting thought. I reckon that if user names weren't there (try to imagine it) I would still be able to tell who a lot of people were, from such things as their posting style, the things they choose to talk about etc. Even apart from that, as an observant person I tend to notice words and phrases people use a lot which can be a giveaway.

For example when we have the Christmas usernames, if I see one and don't know who it is, I'm pretty sure I would know anyway in the vast majority of cases.

Use of smilies and gifs are also big clues if you know your fellow fms reasonably well.

Edit: Adding more: In regards to the OPs point about how we respond to posts/threads depending on who made them, well he's spot on. It's human nature. Not saying its right, but if we are honest we all do it. Some far more than others however, ho hum. If anyone says they don't then they are kidding themselves. I've observed people jump on others for silly reasons, and say nothing on separate occasions when one of their forum "friends" posts more or less the same thing. It's hilarious.