Email as a username has its advantages and it has its drawbacks:<p><pre><code>  - drawbacks

    - spam

    - they change

    - they get hacked, which in turn means your accounts have been hacked

  - advantages

    - easy to remember
  
    - immediate conduit to your user

    - automatic recovery built in (see drawbacks)

    - built in verification of identity (somewhat) of user
</code></pre>
It's a tough choice. We used to use email addresses but of late we have switched to simply allowing people to pick a unique nickname.<p>The deciding factor was the fact that more and more people perceive giving their email address to a service as a serious barrier to participation.