Probably one of the biggest successes of Firefox is its extensibility.
So the next major release of Opera is claiming to support extensions.  From the description, it sounds somewhat similar to how Firefox extensions operate, and they are hinting that they're trying to make porting extensions to Opera relatively easy [...than having to completely rewrite them, maybe].

If it becomes successful, quite a number of users could probably jump to Opera 11.  Just have to see how well it goes.