From what I've heard (from other players, not Blizzard) casual matchmaking also has it's own hidden mmr, so if you haven't played casual too much up to this point or have poor winrate because you test new decks, the system might treat you as a "new" player.
I find Malygos Druid pretty good at those ranks. It can beat everything, but requires some creativity in indetifying your win condition. I feel like most of the people assume that I'm either taunt or token, which ends up as huge advantage for me.
However I haven't played in week, so maybe things changed after nerfs.