Well, Cerebrates basically embodied the consciousness of their individual broods. Queens however direct them from afar. It seems the Zerg are gravitating towards a more individualistic, independent mindset. This is reinforced by the fact that one of Kerrigan's goals is to develop a "Metamorph", a Zerg creature of directing its own evolution.Myself, I always thought the key difference is that when Cerebrates lack a leader, they panick and try to merge into a new leader. Queens are submissive to stronger individuals, but if there is any doubt of who is the stronger one, they'll fight for power until this is settled.
By teh way, I just now got your name, Tche. I feel stupid. :P