Fluff comes first, it is the fairly standard DA fare, which is always a bit cool since they are one of the Chapters that are circumventing the dispersal of the Legions (along with BT & SW, I suspect the Imperial Fists to some extent ("Sons of Dorn"), and I bet the Ultramarines have their own network set up, being the "sons of the politician"). There are a few new tidbits about the conditioning, levels of internal secrecy and arcane tech, including the one I find increasingly annoying...that all the "good" tech is gone and nobody can figure it out anymore anywhere in the Imperium. If true, then there would be no more TDA after 10,000 years, let alone Dreadnoughts, Rhinos, Land Speeders, etc. Probably no power armor either. Now I don't mind the relics bit that are difficult to repair and the need to handcraft replacements while much is actually lost (small & cheap antigrav for jetbikes and Guard speeders), but the entire early "all Marines are fighting with 10kyr old gear" line is getting a bit old. The 10k stuff is the things like artificer armor and master crafted weapons that have been worked over for centuries.
Think about it....20 legions of up to 10,000 Marines = 200,000 bodies roughly. 1000 Chapters of 1000 Marines = 1,000,000 bodies. If they are using ancient wargear, then the story isn't how Horus turned to Chaos and betrayed the Emperor, it is the monumental stupidity of the Traitor Legions, Renegade Imperial Army and various xenos didn't destroy the mega stockpiles of gear lying around the galaxy during the Heresy and Scouring.