Just because it's difficult to remove the "person" from the "message" in face-to-face conversation doesn't mean we can't strive for something better online.
I have a very relevant example! A few days ago I was reading a comment here about someone that drinks tap water and how (anecdotally) they are fine, and they didn't understand the craze with bottled water. Being completely honest, I made a "mental picture" of him as a mid 20's "hipster" silicon-valley type. And I agreed with his overall message. But halfway through his comment, he had a link to his "beverage container of choice" that he drinks water from. I read the whole comment before opening that link. I had assumed that it would point to a glass and leather "artisan" water bottle... The link pointed to something similar to this:
http://assets.suredone.com/1766/media-photos/sd009014-vintag...
After opening the link, my whole view changed. Suddenly that "mental image" changed into a mid 40's overweight sysadmin, and (as much as I'm ashamed to say it), I suddenly "trusted" their message less...
I realized what had just happened, and I realized that just thinking I knew who this person was affected my thinking. If I had known that person's age, size, gender, profession, etc... Would I have given their message a second thought? Honestly I don't think I would have. And that makes me feel like shit. I have prejudisms (is that a word?) that I don't consciously know I have, and if possible I'd like to not apply them. Reading HN comments as faceless-nameless comments helps me read the message with none of that "baggage". And if I later read about the person behind the comment (and learn that it's not what I expected), it can go a long way toward reducing the prejudice that I may have that I don't even know I have.