The no-avatars rule also takes away some of the personalization aspect. If you set your account up with your nickname, your fancy unique profile picture and your favorite quote in the signature, and someone says you're wrong, you're much more invested because you've tied some of your identity to the account.

If you've just arrived on the site, have been given a random name and someone says you're wrong, what do you care? You're not attached to that account at all, it's not "you", it's just a random account on a random website.

I thought that was an interesting point on 4chan (and probably other sites before them), that your identity was set per thread (iirc they only later introduced the ability to have permanent accounts). That removes the possibility of you becoming attached to the random name.