Does it seem to you they have broadened the scope of Deathwatch assignments? Formerly, in the canon fluff the Deathwatch spacemarines were sent out only to combat xenos. From the description for this new book, however, we can glean that now they not just have aliens but also heretics and Chaos to contend with!
This seems okay as combating solely aliens would rob players of a lot of W40K background. However, I do wonder what the difference with Dark Heresy would be, where acolytes also work against these three threats - heretics, xenos and chaos - but I take it the new game will be a lot more combat-oriented.