I would like to ask again for people here to post their photos, as I believe it would help us all here to better get to know who we're responding to. I now it's just an image of a real person, but at least it will help with keeping people straight here, as it's so easy to mix up one name with another, especially when they're mostly make up names to begin with.
Not everybody wants their face shown on a public internet forum, although I see your point about having perhaps more sincere/fair discussion with other forum members, if one gets to see how the other actually look like. However there are several reasons why other people, including myself, would choose not to post a personal photo.
First of all anonymity will make a person feel more comfortable of talking about issues or subjects sensible in its view. Why would a kid with raging face acne put a picture of himself on an open forum, to the risk of getting mocked at by some unworthy forum member(s)? It goes to the person to decide if he can trust the members of this forum to be considerate no matter what written material this person decides to post, or if he has to remain careful and not expose himself too much.
Anonymity also protects you, especially if you happen to be a very attractive woman, against constant harassing from overly crude (male) members. Of course you shouldn't necessarily assume that this situation would ever happen to you, or at least not in such an extreme way that you are then forced to leave this forum. Proof is a lot of attractive girl have made the choice of posting at least one picture of themselves on RPDF. I'm just saying this could be one reason why a person would not post pictures of themselves.
Finally, and something that I find quite amusing in a way, is the gap that you sometimes find between a member's profile pic and what is actually written by that same person. What's more surrealistic than two forum members getting at each other's throat on a chosen topic, while their profile pics show large smiling lads chilling in their backyard, come to think about it?
Of course after some time spent on different social networks you somehow develop the capacity to ignore the absurd break that sometimes happen between mood pictured and written content. Not a very natural thing IMO, but not such a serious issue either. Maybe it's better than no pics at all, if you follow Van's point...