The DuckDuckGo extension you pointed out is really nice, looks like a valid substitute for how I used to search the web. Unfortunately I have to say I prefer much more the way the F2 key worked and here is why:
I had access to the opera address bar, which means the opera searches manager shortcuts. (customizable and faster to type than the !bang system in DuckDuckGo which work only with certain web search engines and have fixed and relatively long shortcuts.)
I like DuckDuckGo and I bet there are many other interesting extensions to search the web, but I prefer much more my older default search engine.
With the 'shift+F2' key I could just type two or three keys on my keyboard and get to a specific bookmarked webpage, no need to open the bookmarks or a bookmark manager.
F2 was just one key, extension shortcuts needs control or alt.
Keep it simple, make it short, easy and fast. This is my philosophy of what a web browser should be, I can just hope that opera web developers will reintroduce the F2 keyboard shortcut in a next release.