Opera used to be superior to other browsers, because it offered a full feature solution - a perfect alternative to Firefox - and it is still the only browser with native tab stacking.
It also used to be the fastest browser for people with an assload of active tabs (hell, the only working browser to that end), and a browser with a decent syncing system. Both of those two are no longer the case; the browser is sluggish as hell - I even have to shut it down now and then, because it hogs so much bandwidth - and the Sync feature is awful (just as it is on Chrome).
There was a time where the link.opera.com website didn't work for something like two or three years(!), and I still can't sync between my Windows desktop and MacBook Air running Mountain lion. A good sync feature is one of the biggest feature that could literally make me switch to just about any web browser. As Chrome's is no better than Opera's - managing the bookmarks locally is an awful experience on Chrome - and porting will be a pain, I am not in a rush to switch.
Opera feels great to use in many ways - it just has so many great shortcuts, and the support for rel="next" on paginated sites is still huge.
Opera keeps innovating and will always do, but it paradoxically keeps ruining its core value proposition in favour of an update schedule that breaks more than it fixes. Don't ask me, just look at community forums where people are sticking with Opera in spite of what it actually is now.
I don't know why it's gotten so bad. Maybe the employees are just tired of working on Opera. It's just such a crying shame for those of us who've used it for for over a decade.
And Chrome still isn't good enough to tide us over.