Interesting character archetypes (aka "classes" in many RPG paradigms) relevant to the setting would include Rouge Traders, Commissars, ElectroPriests, Astropaths, Space Marines, and Inquisitors (and 80+ others, for sure).
- A Rogue Trader is a commissioned freelance explorer, owner of a ship or fleet, with broad social privilege. He may have a crew of thousands at his beck and call. Yet he is just a mortal man.
- A Commissar is a soldier-type leader, also with a fairly broad-sweeping mandate. He can impress the citizenry into military service, and may punish cowardice with execution, no questions asked. He, too, is just a mortal man, no special powers.
- Electro-Priests lack much of that social power and latitude for dismissing fears of accountability. But they can frenzy and shoot lightning bolts, so some may think it's a fair trade.
- Astropaths are blind. Seriously. Their eyes are gouged- or fried- out in a process called soul-burning. This protects them from demonic possession, and ensures their loyalty to the Emperor. They've gotta suck at combat, but they can do tricks like reading minds and telepathic communication with a multi-light-year range.
- Space Marines are more than human. I've already posted about their redundant biosystems. They are definitely not 'merely mortal'. On top of that is ludicrous equipment budgets and the finest training the Empire can provide. Unlike D&D, this isn't some Prestige Class you'd grow into - Marines are genetically uplifted, and trained in cadres that are a blend of extended family and military unit. You could get around some of this by starting as juvenile Scouts, but I think players would feel let down if full-blown Marines are not a starting PC option. Even with a solution to that, Marines have social issues. They live their lives knowing exactly what is expected of them, and anticipating little freedom and no retirement. Even death may not preclude their continued service to the Emperor.
- Inquisitors are much like Space Marines. They're also psychic, with some really awesome offensive powers. They have social status far exceeding any of the other character types I've mentioned. Pretty much, they get everything.
I'm really curious to see how the upcoming game makes it all work. I understand the first book (Dark Heresy: The 40k RPG) casts the players as the entourage of an NPC inquisitor, but two other corebooks are planned to handle other conceptual paradigms.