Sidebar stacking tabs. Firefox has a plugin that emulates this functionality Opera offers by default, but the implementation is pretty bad. After all these years and especially after the widescreen migration, one would expect to see browsers offering more vertical real estate by placing tabs on either side of the window. At 1920x1200 resolution it works absolutely perfectly.
Firefox has pretty much fully copied Opera's database-style bookmark system, so if they ever offered this functionality natively I would migrate. IE9 has some tab options but doesn't appear to offer it either.
I am simply tired of the idiosyncracies due to residual bugs in Opera, as each major update adds a few more without fixing most of those that already existed. Such as typing a word into the bookmark search box, and nothing happening. Or clearing the box after searching, but the window still shows the previous search results (it's a realtime keyword/title/metadata search system for the bookmark database, updates are reflected instantly when searching). At least the "bookmark all open pages" option actually creates the bookmarks in the selected location now, instead of an utterly random location in the bookmark directory...