The story is feeling stagnant now. I know the Age of Darkness has a lot of time to cover, but it feels as if all the momentum has gone out of the series since Calth. Back when the series started, the main events I was expecting, were:
Davin. Istvaan III and V. Prospero. Calth. Signus. Mars. Terra. Possibly Tallarn as well, but I never really considered that to be a main event like the others. Those were, and are the main events. Then we got some nice add on novels as well, such as The First Heretic, Legion, Scars. Which were great, helped pad out the precursor to the Heresy and fleshed out two of the more mysterious Legions. We also got the Guillimans, Imperium Secundus, which wasn't executed brilliantly (too much Curze), but still works into the story.
But now, it's just so many other stories that just don't have enough substance or background to the event. All those main ones are covered, everything else apart from Terra, now has to be almost completely new material and filler. Other than Mortarion and the Death Guards fall, there aren't any events from the days of Index Astartes articles or Visions to look forward to.