Isn't that a regression? How to tell users apart if you put the same brand logo on all of their profiles?
In any case, I really hope they return. There is the username, of course, but it is a nice touch to have a mostly unique simplistic avatar for those who don't supply their own profile picture.
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Update: It looks like the Identicons might be back.
Should we all stop using ASCII fingerprints for keys because it could accidentally say e.g. 6 6 6 / 212 = 3.1415?