I've just installed Opera 20 yesterday, and after couple hours of fidling, I found it awful. It turned into Chrome, but with every bad thing about it that caused I didn't use it. I've used Opera since 2006, and stayed loyal to it ever since, and... it should just focus on further improving Opera 12.
First of, I loved Opera for having minimalistic approach, with cards/menu on one line, and below the url bar with search. When you clicked it, a third bar appeared with bookmarks. In 20 there's no way to customize it, it took a while to find how to make bookmarks appear, but the third bar takes just so much space, and it won't hide. That's a huge minus and step back from the big space 12 offered.
Then you have the starting page. In 12 you could easily customize, the google bar wasn't' necessary, though it was small and you never paid any attention to it. The windows to sites (I'm sorry, I don't know their name in English) were easily available, and you felt that the quick start page was all about them. In 20 you have a huge google bar with unnecessary options, and the windows are, well, you can't customize them at all. All the options were removed, as if the devs thought we were too stupid to use them. You CAN'T customize the looks at all anywhere (outside the background of quick start. Even more, now in 12 it automatically changed the size of windows, and in 20 they forgot to implement it, and instead of quick start page, you have scroll down for more page. Ugh.
Another nifty future of previous Opera version was opening a window in the same page. Many links try to open in a new card, but right clicking the link and choosing "open" would open the link on the same page. In 20 it's gone. Why? No idea, maybe because Chrome also didn't have it? Or dunno. It just makes browsing more annoying now.
There are also such small things like, hovering over a link, it would show the url next to the cursor after a while. Now it's on the very bottom, just like every other browser out there.
There was something else I've spotted after couple hours of using 20, which really annoyed me and was a step back from 12, but I've already forgotten about it. All in all, I really don't want to use Chrome 20 ever again, and will just wait for Opera 13 with a new engine or something, that would be more compatible with more demanding sites.