Redeucer said:

Oh, I'm well aware of that - the White Wolf/WoD example was just the one that sprung to mind first. As it stands, though, I wouldn't exactly count Savage Worlds, GURPS, the Hero system or D&D in the same vein, though - 40kRP and World of Darkness are alike in that there's a distinct setting that the rules are written for... while Savage Worlds, GURPS, Hero and D&D are all (to varying degrees) setting-neutral or otherwise generic. The Mongoose version of Traveller is similar in this regard - the main rulebook contains a few nods to the Classic Traveller Universe, but the game is presented and marketed as a generic sci-fi RPG.

Personally, I can't touch a generic RPG system without feeling compelled to significantly adapt it to fit the specific nature of the setting I'm playing in (and sometimes not even then - I cannot stand GURPS, and I've never been inclined to touch Savage Worlds or the Hero System). I can't play generic or setting-neutral games 'out of the box' so to speak, and while this is an asset at times (I've done plenty of work tweaking and setting-tailoring d20 Modern over the years), I measure the quality of a system based on how little I feel compelled to houserule (remembering that there's a distinction, albeit a fine one, between houseruling to make something work more appropriately for a given game, and houseruling to add new or more detailed content that isn't necessary). In my opinion and experience, the broader and more generic something attempts to be, the blander and less interesting it is...

So, in that regard, Dark Heresy (and the model established by Dark Heresy) works because it isn't generic, because there's an assumed basic setting to which the rules have been tailored (with means enough to deviate from it if necessary with a little imagination)... for me, a generic 40kRP would have been appealing mainly because it's a 40k RPG, but Dark Heresy sits alongside WFRP and Fireborn as one of my favourite RPGs ever, while I doubt that a generic 40k RPG (and I've seen a number of versions over the years, including a couple of my own attempts) would be anywhere near as satisfying to my particular sensibilities.

There is the other angle as well - rule refinement. RPG rules can only change so much through errata, before the errata and FAQ sheets end up being worth an entire chapter in their own right, and the natural lifecycle of most hobby games seems to be half a decade or so... as GW demonstrated with the refinements made to their Lord of the Rings wargame between the first three editions only a year apart each, a game system can be more thoroughly and more effectively cleaned up and smoothed out with more closely staggered releases. Rogue Trader has the same potential here - the game isn't a new edition, but it's an opportunity to clean up the rules in a more significant manner than really feasible through errata and FAQs alone. One rulebook and an array of setting books lacks that advantage, though how important or significant you consider that advantage to be is, obviously, a matter of personal taste.