Area 51 uses a different system for avatars than the rest of the network. While on every other site Gravatar URIs are salted, on Area 51 they aren't because it hasn't been updated (Area 51 uses a really old fork of the SE engine). This could allows determining the email address associated with any user who has an Area 51 account. Avatars on Area 51 should be salted to prevent this.
Area 51 avatars should be salted the same way as the rested of the network to prevent this information leakage.
See also this deferred request for the whole avatar system to be updated (I don't want a whole revamp, just this one bug fixed).