I disagree. The Walking Dead is a character study -- it's a post-apocalyptic show about survivors struggling day-to-day in a world that's gone to hell, and while, yes, some of the characters haven't developed much at all, the ones that have are all brilliant.

The most obvious choice here is Rick. During Seasons 1 and 2, he was a man that lived by a moral code. He went back to Atlanta to see if he could find Merle, looked for Sophia after she went missing, went back to get Shane (who he had just had a fistfight with) when he was trapped in the school bus in 18 Miles Out, and struggled with the decision to kill Randall. As time went on, Rick became colder -- he didn't hesitate to put down Tony and Dave in Nebraska, stabbed Shane in Better Angels (which lead to the beginning of the Ricktatorship), killed Tomas in Sick, attacked Woodbury and thought about handing Michonne over to the Governor. Eventually, he went from somewhat erratic to cold. While Season 3 Rick was a little all over the place (and I mainly blame that for Mazarra's doings), he has developed tremendously. Take a look back at Season 1 Rick -- he's completely unrecognizable, as are many of the characters that are still alive on the show.

Another example would be Carol. She went from being an abused, weak woman that was afraid to even speak out of turn to a woman who is strong, sure of herself, and brave. She did a complete 360.

Hershel went from being a sheltered man who believed that the dead could be saved, to a caring member of the group who did his best to make things right.

Shane went from being a strong, capable leader who wasn't afraid to make tough decisions to a broken, lust-filled man which ultimately ended up being his downfall.

Daryl has changed from badass redneck who was slightly one-dimensional in the earlier seasons to a flawed man with a heart of gold.

Carl's gone from a naive, sweet little boy to a teenager who isn't afraid to step up to the plate when the situation calls for it.

Beth went from being a suicidal teenager/designated babysitter to a girl who wasn't afraid anymore.

Merle, while always an asshole, changed from being a redneck who didn't care about anyone except himself to a man that realized he needed to do something right in one last moment of redemption.

My point here is, a lot of the characters have changed. While there are an odd few that haven't had much development (Tara, Rosita, Eugene), a handful of them have developed leaps-and-bounds since the early days. While sometimes certain character's behaviors are a little erratic (Lori and Andrea anyone?), most of them have been pretty consistent ever since Gimple took over and they've only been becoming more multi-dimensional as time has gone by.