Another standard feature missing in Opera's built-in mail client is support for digitally signing and encrypting/decrypting emails.
Signing a message is important so the recipient knows it was really you that sent the message. Encrypting the message ensures that no one can see the content of the message except for those that have your public key. It's a great validation and privacy win (that's often required for employees in their company).
Many mail clients like The Bat!, Kmail, Sylpheed, Claws Mail and Thunderbird allow you to do this. But, Opera doesn't support this at all.
Now, your average user might not care about this, but for those that are required to use it (or those that feel it's essential) they're just not able to use Opera's built-in mail client, at all. 🙁