2.7+

The problem is that WP’s default Mystery Man gravatar (used when commentors have not registered an avatar for their email address at gravatar.com) is rather ubiquitous, making most WP installs look a little bit too much the same. The other WP options for the default gravatar (autogenerated unique Identicons, Wavatars or Monsters) are not to everybody’s taste, and certainly don’t fit into every design concept.

The solution is to create a unique avatar just for your site, one that suits your design for a professional site or perhaps one that references an injoke at a personal blog. Either way, it will make your site stand out.