With almost a year left of development, one would think that there is a lot left to do on Batman: Arkham City, but according to an OPM podcast (via System Link) the game is "finished", and all the content is there and it's playable from start to finish.
What remains is a lot of polishing, side missions, and effects such as rain. Let's hope all this time for polish results in an experience that's on par with the original.