While at GenCon I got to speak for a while with one of the developers of the game. We talked for a while on system and differences and similarities between DH and WFRP.
One of the core differences is that while in the Old World, you can start off as a Rat catcher, and work your way in to becoming a Knight of the Inner Circle, Wizard Lord or High Priest of Ulric...the adventurer in the 40k universe doesn't have the same social-climbing ability. You start off as a Hive ganger juve...and in 20 years, at the end of your life likely, you'll still likely be a Hive Ganger.
In the WFRP system there is a lot of career-hopping, but little in the way of depth WITHIN a career for diversity. If you want diversity in WFRP, you change careers.
In Dark Heresy however, this is handled quite the opposite.
In Dark Heresy, the Character careers will have a lot of customization available within them to make a wide variety of characters, within a single career. You may be a former guardsman, but now you can tool out that guardsman career in to becoming a cool sniper guy, or a close combat monster, or have depth in any number of ways...within the same career.
Also, weaponry in 40k Dark Heresy is going to be very lethal. A Bolter round will very likely kill you. A Meltagun will almost certainly kill you. A lasgun can most certainly kill you. Everything...can certainly kill you. Remember, 40k weaposn are based on Marine Equivalents, and for the most part (at least for a bit) nobody is going to be a Space Marine. There IS a Deathwatch book expected next year though.
I believe the expectation that the adventures will be largely investigatory, searching for and hunting down cults and heresies as junior retinue for an inquisitor and that sort of thing, and combat will be the garnish, not the whole meal (and in a universe as deadly as this, believe me, you don't want to be a mere human in combat with these weapons, in these environments).
Thus far I'm stoked about the system, and the setting. A lot of the old familiar parts of the system will remain the same. The core mechanics are the same. I can't wait till February!