I miss so many things from the old Opera browser, but I guess my favorite features was panels (and the possibility to show your tab bar on the side of the screen...)

I have an 16x10 display and when I'm surfing the web at home, I'm usually just doing that. I don't have another window opened that I want to arrange side by side with Opera. (It's a different story when I'm at work, but I can't use Opera at work anyway)
Because a usual website only uses about 50% of my available screen space, I loved the Opera features that actually made perfect use of the additional screen space:
-Having panels as a side bar (especially for showing information about downloads)
-Show tabs on the side as thumbnails

This is nothing an extension could solve in a way Opera 12 did.
(Having panels back would even blend in great with the additional extension availability, if it would be possible to have a panel page for each extension...)