Another standard feature that Opera is missing is an indicator that shows the insert position when you're dragging selected text in a text field. Try this example.
As you will see, Firefox, IE, Chrome and Safari will all have an indicator showing where the insert position is in the textarea. Opera on the other hand just changes the mouse cursor to a + sign, which makes it very difficult to get the insert position correct. ('+' means "copy" btw, which is wrong in this case as you're moving the selection.)