The Galaxy is a large place. On some worlds, an Imperial muster is bound to take place, and it seems that this exhibit demonstrates that on a very grand scale. This diorama is of a parade ground for the Imperial Guard (or Astra Militarum as they're now known).
The mighty Hammer of the Emperor marching and rolling forth is being overseen by a combination of senior ranking officers of the army, as well as the Imperial Fists chapter of the Space Marines. If you look closely in the pictures, you will even see an Imperial Fists rhino parked at something of an odd angle to the side of these proceedings. I like this touch in particular -- the presence of the Sons of Dorn shows some connection to the Imperial Army that I'm not quite sure of (are they being subservient to the Army, or are they in command of the Army? I could not make up my mind on which way around the chain of command went from just looking at the display - so let me know if you have an opinion).
All around the diorama are the impressive vehicles and walkers of the Army. Of course, there is also the Imperial Guard themselves. If you scour the pictures carefully, you'll see that they are for the main part marching in exacting lock-step with each other. This is an impressive bit of posing guardsmen on a very large scale that takes the breath away.
In the background to all of this are large scale weapons that are pointed at the skies. Such macro cannons are indicative of a very fortified world (like Cadia!) that I was particularly pleased to see. I think my only criticism is some of the plain steel walls at the back of the diorama -- they've simply attached some cogs and pillars to the front of these, but I might have liked to have seen some segmentation on the chrome-looking pieces.
There are plenty of other "Easter Egg" style things in the pictures as well. For instance the hanging flags are particularly good and (to me) even scarily reminiscent of war reports that I have seen on television in our own world.
Very impressed with this diorama -- full marks for layout and execution. And bonus points for being intimidating and reminiscent of real musters.