Wrath&Glory has attempted to capture a huge range of 40k gameplay in one book and managed just that! You can play lots of archetypes from lowly imperial guardsman to deathwatch primaris captain! Inquisition, Hive Scum, Mechanicum, Chaos Cults, Eldar Corsairs and Orks - everything is playable straight from the book. There are even rules for Vehicles, Void Combat, Eldar Psychic Powers and Ork augmetics. To sum it up, Wrath&Glory captures a huge variaty in a starter book, which other systems would take years and 20 supplement books to achieve.
The game mechanic is interesting and combines typical hit&damage combat with some special twists. In particular, the precious Wrath Points can be used by players either to reroll all failed dice on an important roll (pure power-game), or to make a narrative declaration and changing the course of the story without rolling anything at all (pure roleplay). Since you gain Wrath Points mostly through good roleplaying, the system motivates you to stay in character, even if deep inside you are just after those juicy rerolls :)
The book covers the basics of post-Cadia 40k fluff, but does not offer a lot of depth in the setting. Instead, it offers you narrative hooks and ideas to base your campaign around, whatever your playstyle and group composition may be. I expect that expansions to this game will mostly focus on adding more story content for different character archetypes, rather than introducing lots of new rules.
P.S. Looking forward to the dark mechaincum book, if its ever gonna appear! :)