Another standard feature missing in Opera's built-in mail client is support for pasting images from the clipboard into the body field of an HTML message you're composing.
In Thunderbird, Evolution and Eudora for example, if you copy an image to the clipboard, you can focus the body and hit ctrl + v to insert that image into the HTML body. This will embed the image data into the source of the message as a base64-encoded part with a content-id specified. Then, the src attribute for the img element in the HTML markup will reference the attachment via a cid: URI that references the content id.
Opera doesn't support this at all.
Thunderbird even allows you to drag and drop a file from your desktop into the HTML body to do the same. In Opera, this always just attaches the file even if you're dragging it to the HTML body field.
There's a thread about this here.
On a side, Firefox does something pretty cool. It supports pasting images from the clipboard into rich text editors on a web page. When doing this, the image data is embedded in the src attribute for the img element as a base64 data URI. Other browsers don't support this though.