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In addition to his work on The Shield, Mazzara was showrunner on the STARZ L.A. drama, Crash. He was replaced for the second season. I have no idea what this says about his showrunning abilities, but I'll defer to Darabont, who knows what he's doing (expect for that ridiculous hospital bed POV from the first episode of THE WALKING DEAD).
Online fanboy negative response notwithstanding, there's absolutely no reason to declare this the end of THE WALKING DEAD. Darabont is a filmmaker, not a showrunner, and now that he's gotten THE WALKING DEAD off the ground, after five years of trying to bring zombies to prime time, it makes sense that he'd move on to other things. (Like maybe a film version of Stephen King's THE LONG WALK.)
This changing of the guard has not affected the production of season two of THE WALKING DEAD, which is set to premiere on October 16th.