There ARE some very serious concepts, factions and stories in there, but of course there are over the top, ridiculous things and a bit of comedy to relieve the tension. The Orks, Squats, Gretchins, and a few others are there to just be funny (while still dead scary).
Here's the real problem I see: The poster boys are always the boring ones. The Ultramarines are bland, faceless, emotionless and in a lot of ways seem brainless. They do everything by the books and demand everyone else do the same. Here's another option: The Black Templars are an existing chapter of Space Marines that are up to eight times regulation strength (potentially 8000 strong) on an endless crusade spread throughout the galaxy. They employ lightning fast storming tactics and better individual troops to smash through and break enemy lines. Sounds much more interesting than an army of bland Judge Dredds in power armor, right? (Not to badmouth Judge Dredd, my point is that he's by the book to a fault). Hell, Space Marines aren't even the most interesting things to see. They're inhuman. They have no relatable feelings to the human race anymore, if the fluff is to be believed. They are BEYOND human, but guard it with righteous fervor.
I have a proposition: Make a game about being an Imperial Guardsman. Make it a game about being a redshirt fighting against impossible odds, fighting battles no one is meant to survive, winning on the skin of our teeth against monsters larger than tanks with claws that could tear down fortresses. The Guardsmen who aren't totally brainwashed are colorful, alive, they have interesting backgrounds and histories. You could make characters out of Guardsmen. Space Marines are rough killing machines who are self sacrificing to the point where most of them wouldn't have anything relatable to say.
Another faction I'd want to play as is the Craftworld Eldar. There's an interesting duality between Eldar and Humans, some believe they are better than humans, some think they are similar enough to be trusted. Many believe Eldar to be the most potent beings in the galaxy, but they created their own demise by becoming TOO powerful. An RPG where you could play as an Eldar (in the style of Mass Effect's storytelling or something) could be a good premise for a game. Plenty of skill tree paths, career paths, and moral quandaries.