The show has a lot of problems but alas, it still has zombies.
I think the main problem is that the show stretches itself out trying to market itself to too broad an audience.
For instance there is a lot of drama to reel in the ladies, etc.
The premise sets up several types of post-apocalyptic tension: (1) tribe v. world/ zombies; (2) intra-tribe conflicts; and (3) inter-tribe conflicts.
Each season has moved onto the next type of conflict. My wife, for instance, liked season 2 the best because of the love triangle between Rick, Shane and Lori. Those sorts of intra-tribal politics were hugely significant in primitive human societies, and they remain compelling due to evolutionary psychology.
In other words, I don't think the producers are stretching to broaden the appeal. The strength of the show's premise is that it can convincingly create tension in a lot of different ways.