Needs more grimdark.
Just from a technical standpoint it jumps around a little bit, and the definitions of the characters and setting need some work. For a good summation of 40k you need only look to the front of every 40k novel. The opening scrawl paints a vivid and engrossing picture, and compare and contrast it with the HH opening and you can almost see the grimdark starting to leak out of the binding.
I would start off with something more along the lines of, "While Warhammer 40,000 is at its basic a table top miniature strategy game, the true lure of 40k as it is colloquially known is its expansive lore providing the back drop for the one of the most grim and dark portrayals of our future."
You can hit on major points such as the Emperor subtly directing man, the Dark Age of Technology, Age of Strife, Great Crusade, the Heresy, Age of Apostasy, and some of the more notable battles of late m41. I'm a fan of compare and contrast, so stuff like, "OMG Orks! Never fear, Deathwatch! OMG Daemons! Never fear, Grey Knights! OMG Heretics! Never fear, Sisters of Battle!"
