Another standard feature missing in Opera's built-in mail client is support for importing and exporting an address book in csv and vCard format. Most clients at least support these two.
Opera only supports importing and exporting in its own adr format. This makes it difficult to get addresses in and out of Opera.
There are workarounds for Opera though.
* If you have a my.opera.com account, you automatically have a mail.opera.com account. The webmail there supports importing and exporting in all kinds of different formats. You can import and export there to convert address books to/from Opera's format to other formats like vCard and csv.
* If you export another mail client's address book to csv, you can convert the csv file to Opera's adr format with <http://hallvord.com/opera/conversion1.htm>. You do have to manually pick what fields you want etc. though.
* If you export Outlook Express's address book (all fields) to a csv file, you can use the Outlook Express Abook CSV Converter Opera widget to convert the csv file to Opera's adr format. In this case, because the widget is specific to Outlook Express, it can convert to adr format without asking you anything about the fields.
But, all of those are annoyances as they're not very discoverable and they're not built in to Opera. Users have really had a problem moving to Opera because of this. Luckily, those workarounds exist.