I like identicons, but too many of them together is just noise, for example:
My eyes just want to leave that area without even looking at it. And can you blame my poor 
?
Yes, it is definitely better if every user has an avatar set, but how likely is that to happen, here, or everywhere else in the world that Discourse is installed? We need sane, safe defaults that work everywhere with minimal effort.
I believe a solid color + letter would look a lot cleaner there.