But, they kinda nailed it with Monks imo. I think that monks are already in a place that took DK's nearly two years to get to. So there isn't a giant mess of a new class holding them from adding another.
Overall class balance will -forever- be an issue. There is no chance in hell that they will ever perfectly balance all classes/specs, so that's hardly an argument against adding more. I think that as long is class balance is in a fairly good spot, there is nothing wrong with adding another class.