Mostrar usuarios como “Dominio\NombreUsuario” en vez "Nombre para mostrar

In most cases it is the Profile Sync job which is turned off and as a result the synchronization doesn't happen.

But sometimes even the profile sync is on, but SHarePoint 2010 can't update the accounts. In such a case one can - Powershell Command "Set-SPUser".

1) If the problem only appears with a single user, you can update a single account like so:
Set-SPUser -Identity ‘domain\Username’ –Web http://<URL To Site> –SyncFromAD

2) If all (or a lot) of the uesrs you can do it by the following:
Get-SPUser –Web http://<URL to Site> | Set-SPUser –SyncFromAD

The above will reset all users FOR THAT SPECIFIC URL; if you have additional site collections (i.e. Managed Paths), you have to run it on those URL's as well.