Hay guys, I say a Doomrider box at my local hobby store. I can still use him right?!
Doomrider! Na, na, na, na!
Older characters have rules at Games Workshop's website, though for some reason GW claims they are 'experimental' characters, rather than 'retired' characters, and they all require your opponent's permission to use. In the case of Aun'Shi someone at GW decided that characters should enhance the strengths of the armies they are in, instead of make up for weaknesses. So either the Tau were surprisingly good at close combat and Aun'Shi was a bonus, or he was making up for the Tau's lackluster close combat performance.
Rather obvious which one of these GW decided he was.
Farsight didn't get borked because he was popular for conversions.