We're in the middle of upgrading our avatar uploading/selecting service. That'll be here soon, but as part of that, we wanted to stop serving the default "Blue Gravatar G" avatar as the fallback when a user doesn't have an avatar at all. Now we have a new default avatar:
It's a bit less aggressive in the grid view:
And more obviously an avatar on profiles:
Hopefully it's not too confusing of a change (we saw a little confusion on Twitter). There is no deep-seated meaning here or sea change.
Ideally, of course, you have a real avatar.