One thing I wonder about is whether the American public is going to be willing to stay with a show this grim for the long haul.
Most people are used to shows that have neat endings and even mythology series (Lost, the X-files, etc.) have moments of joy and/or levity to break up the drama and the scares. And a typical horror movie is usually over after only two hours.
TWD, on the other hand, while brilliant, is an unrelenting tale of dystopia, tragedy and horror. I'm not sure that an audience that expects happy endings is ready for five or more seasons of a story that makes Cormac McCarthy's 'the Road' look like an episode of the Courtship of Eddie's Father.