Tile and autofill
The two things I really miss form opera are which I hoped we would have back by now.
1: The tile feature. this was great for dragging information form one tab to another which I use a lot with such things as on line competitions etc.
2: The original forms fill option. There are a few add ons for this but they over complicate everything by trying to fill the whole form in at once which never works correctly and I spend more time correcting what it got wrong. The old forms was great just typing the start of a word in a box gave a list you can choose from. Much better.
Its these 2 things that means most of the time I have to resort to using opera 12.15 which do both these things perfectly.
I really the tile feature as well, it was what I primarily used opera for - I had two nagios status displays I could have side-by-side, and now I can't. I'm probably going to reinstall the old version.