The biggest annoyance with Opera for me:
when I`m accidentally clicking the wrong link, the new page (wrong one) is being loaded and I must wait for it to load completely until I can move back to the previous page. That`s because on fullscreen mode back arrow is greyed out - it`s clickable (solid) only when page is fully loaded.
When operating in "window" mode (fullscreen=off) I can click that arrow in any given time, stopping loading of the new (wrong) page and returning to previous one.
I would really like to have a full screen mode back arrow to behave like that one in "windowed" mode.