Another standard feature that's missing in Opera's built-in mail client is support for moving and copying messages to a specific folder via the right-click context menu.
In Thunderbird, Outlook Express, Sylpheed and some other mail clients, you can right-click on a message or selection of messages and choose "move" or "copy" to move or copy the message from one IMAP folder to another. Choosing move or copy gives you a list of folders so you can click which one you want to move or copy to. This is a very simple and easy way for users to move/copy messages around.
Opera doesn't have this and you can't even create a custom menu setup for this. Instead, you have to rely on dragging (move) or ctrl + dragging (copy) from one IMAP folder to another. Or, you can use ctr + x (cut) or ctrl + c (copy) and ctrl + v (paste) in the source and target message lists for the folders.
It's great that Opera supports those *extra* ways to move and copy. But, it's not very friendly to users that Opera doesn't support the standard way like other clients.