One (if not the only) commonality among identities in cyberspace is the inclusion of one's Internet service provider (ISP) in one's identification. The epithet after the "@" is all-important; without it, one's a nobody (nobody that can be easily found, anyway). In addition to Internet access, each ISP provides all its users with a set of connotations. The strings "brown.edu", "whitehouse.gov", and "aol.com" all carry specific connotations that allow one to make assumptions about a particular user who operates from that network.
In the last case, a strong prejudice has arisen; on Usenet, one can find a flame newsgroup called "alt.aol-sucks". To many, to make assumptions about an individual based on his/her ISP is as misguided as making assumptions based on gender or race; others argue that if users don't like the connotations associated with their ISP, they can select another one.
Whichever ISP one selects serves as the one unalterable identifier in cyberspace. One's name, one's gender, one's race can all be changed at whim, but everyone will know where you're logging in from. (Certainly, it's possible to attempt to disguise this, but savvy users will see through the subterfuge.)