The answer, as Season 4 [began last] night, is quite possibly and intentionally “nothing,” which would stand to reason: AMC’s zombie saga is a hit because it is an exercise in hopelessness and even nihilism.
With “The Walking Dead,” there is no goal in sight. We are not building toward payoff. There is only a reset button (revolving showrunners, new writers) or a fresh level of horror and despair, just like the popular video game based on the show which emphasizes the feelings and personalities of characters, who could all die at any moment. The TV show also banks on non-zombie intervals of personal conflict among the still-living, who behave like test subjects in a morbidly long experiment of applied social psychology.
Source: The Washington Post