I love Opera 12.16 (last 12.17) and used it loooong time. Many times I tried other browsers and since 1 month working on new Opera (ver. 25). Just a few extenstions (e.g. Classic Tabs, TabHamster) , change some experimental functions ( opera://flags/ in address) , changes in opera settings - and now new version is not so bad. Many shortcuts and gestures are the same (not all unfortunetly). I recommend to try it.
likely some ad or script that's `sometimes' present and that the old opera is struggling with - i've noticed it showing its age at times myself. i've even noticed some sites that i use the extension lastpass on, will not input the password. i think lastpass is slowly updating itself but has lost interest in the unsupported presto engine. these issues will continue to grow more prevalent with time. i've been forced to use other browsers for those sites:(
I've resolved my issue. After starting this thread I realised that I had my dad's laptop here and he also runs Opera 12.16. I tried playing the embedded video on his computer and it worked, as did the other embedded videos that I've been having a problem with. So, I backed up my bookmarks, speed dial settings etc. and did a completely fresh install of Opera. Even though I am back to where I was before, with the same settings and same extensions etc., the embedded videos now play on my PC too. I have no idea what caused the issue and nothing I tried, outside of a full uninstall/reinstall would fix it.