As you see below, in opera browser Google + games says that:
We’re sorry, but it appears that your browser does not currently support the web technologies needed to play Angry Birds.
To teach the pigs a lesson: Download Google Chrome, a fast new browser – now with the Angry Birds game. (Recommended)
But no, i have not suprised. Google still don’t identify opera as one of browser that “support the web technologies” . But google don’t know, in the end the winner will be opera. I remember google wave…