Things that broke with the Opera 15 release due to the switch to Blink/Chromium
Opera 12 vs. 15 comparison on caniuse. http://caniuse.com/#compare=opera+12.1,opera+15
I was surprised to see large parts where the 12/15 feature lists are disjoint, which is exciting, as hopefully the v12 features will be ported to 15, and they'd be really strong.
As much nostalgia as I have for the Opera of the olden days, of which I was a long time fan, I really like v15, using it now. Once they catch up from the shift I think they're on their way to being the innovation leaders in browser features once again.
Most of those are pretty obscure or minor issues compared to the lack of support for Linux. At least people on Windows or OS X can use this new version of Opera, even if it is broken in many ways. Linux users don't even have that option.
You can link to stylesheets in HTTP headers?
What's the benefit, faster page rendering?