Prejudice and stereotyping rear ugly heads when people talk about demographic diversity: race, gender, age, national orientation, religion, sexual preference. Diversity measured and promoted that way may completely overlook and even homogenize diversity of how people think.
In law departments, how people think makes far more of a difference in their effectiveness than any physical characteristic. (See my post of Jan. 6, 2006 on demographic and social network diversity.) Cognitive diversity isn’t a Constitutionally-protected category, but it is vitally important.