This Plugin replaces the get_avatar function in pluggable.php to pass an alternate image to gravatar.com that generates the user’s display_name if they do not yet have a gravatar.
More features coming soon…
Plugin Updated Nov-17th

Display Name If No Gravatar

This Plugin passes an alternate image to gravatar.com that generates the user's display_name if they do not yet have a gravatar.

Jason Coleman

Paid Memberships Pro

Kim Coleman

Andrew Lima

This plugin hides the “Display Name” field on the Edit Profile screen for all users. Instead of allowing users to set this field, the plugin will always set the User field display_name to their first and last name. If these field are empty, display_name will be set to their username.
Display names are set when the user registers as well as when a user’s profile is updated via the WordPress admin.
The plugin includes a batch process to update the display name for existing users. Navigate to Settings > Force First Last in the WordPress admin to run the update.

Force First and Last Name as Display Name

Thank you for this simple and effective solution. It is something that should be part of WordPress by default.
Cheers

Simple and effective

Force the user field "display_name" to be set as the user's first and last name.

Display Name If No Gravatar vs Force First and Last Name as Display Name