It started out with some interesting concepts, a suitably dark re-imaging of Oz. Less song & dance, more realistic conceptions of a world in the thrall of a wicked witch. The Scarecrow himself was compelling enough to counter Dorothy's flat performance. Some of the problems were evident early on - the witch's tendency to pout and stomp around seemed a bit out of character, and any man, tin or otherwise, who spends years locked motionless in an iron suit watching an infinite loop of his family being tortured and killed does not come out as unscathed as this one. But then, he seemed pretty much immortal, didn't he?
Things started to fall apart in the second episode, or rather, the illusion that things might be hanging together starts to fade, but there's still hope. But the disneyfication effect had become really visible by this time. It's like they'd been commissioned to write a good, dark vision of Oz, had some great ideas to that effect, and then the studio came back to them and said, "This is really good stuff, but it seems a bit dark. And we want dark, but with a positive feel."
But the part that really made me want to wash my brain out was when the Wicked Witch was revealed to be a Dalek.