Today's standard feature that Opera is missing is positioning the cursor or selecting text by clicking a rich text editor's padding.
Try the demo. In Opera, you can't position the cursor by clicking on the padding at all. Works fine in Firefox, IE, Chrome and Safari.
Also, selecting the editor's text by starting the selection in the padding area doesn't work very well in Opera. It's difficult to get all the text selected unless you shift to a straight horizontal drag when you get most of the text selected. This works great in Firefox, IE, Chrome and Safari.
This issue makes it a lot harder to use a rich text editor in Opera. It not only affects editors on web pages, but affects the compose body and signature editor in Opera's built-in mail client.