With the arrival of the first Warhammer 40,000 roleplaying game, Dark Heresy, RPG.net is abuzz with ideas for gaming in the setting (and in case you missed it, future 40K RPG products will come from Fantasy Flight Games rather than Black Library). Although Dark Heresy is about playing the retinue of an Inquisitor, one player's question about the Tau convinced me that, were I to run Warhammer 40K roleplay, I might well run a Tau campaign.
Warhammer 40,000 is one of those settings that you basically don't want to live in. Our human heroes are ruled over by a particularly frightening, fascistic take on a dark-ages Catholic church if the Church worshipped a decrepit, people-devouring, largely inanimate God-king who mainly serves as a locator beacon for interstellar travel. The other civilizations aren't much better.
That said, the Tau are kind of on the upswing, moodwise. Sure, they have a rigid, caste-based system and an overriding devotion to a central cause (it's 40K -- who doesn't?), but they're portrayed as a young, optimistic race out to conquer the universe and not quite yet aware that the universe is full of horrid things in balance with each other.
I think it would be great fun to have the players represent a Tau exploration team, boldly going (for the Greater Good) where no Tau has gone before, and encountering dying races, the devouring Tyranid hordes, death planets, rampaging Orks, and the unending legions of the weird, backwards-yet-universe-dominating humans. The different castes and their roles provide a range of niches for the players. In a way, you could run it as a hybrid of Battlestar Galactica and Star Trek, with the hopefulness level set somewhere in between the two. Send your Water Caste crew down to explore the surface with a Fire Team in support, then figure out what to do when the weird, undead robot things occupy your landing craft! Explore a derelict alien spacecraft and attempt to operate its unique interstellar transportation gateways! Solve problems in five minutes following a forty minute setup!
And so forth. I think it would be a lot of fun, especially if everyone got into the "working for the team" mentality of the Tau.