"Have to" be represented sounds like it's a chore. There are many ways that people can be diverse, I wouldn't see the need to "stop". But if I had to think of a list I guess it would include:

Age

Race, gender and sexuality*

Nationality and/or experience being the Other

Favorite programming language

Economic background

Religious background

Languages spoken

Regional identity

Personality

Fashion, in a broad non-prescriptive sense

Promises vs. callbacks

I spend way, way, way more time working with my colleagues than I do directly determining the makeup of my firm. But I hold the thesis that diversity and success (particularly creative success) are correlated, and apply that thesis in deciding where to live and work.

* Particularly in the way that everybody forms their own person out of their genetics/biology and the surrounding society.

What evidence do you base this thesis on? Do you have examples of say, successful and racially diverse companies?