Both men and women have qualities that are both over expressed or under expressed. When a woman takes on the qualities of a man in a leadership context, forgetting her natural feminine gifts of empathy, listening, or openness, she becomes imbalanced in her approach, and thus not as effective as she might be. However, these gifts and capacities that women bring to the table, when fully expressed and balanced with more masculine traits like being direct or decisive, make them better overall leaders.
There's lots of data to support this now and it's up to both men and women to embrace the qualities that make them more balanced. Read Tony Schwartz's opinion about this topic here.