Warhammer - and its daughter setting, Warhammer 40.000 - is generally known for being "grimdark"; that is, "in the grim darkness of [the far future/fantasy Germany], there is only war, disease, starvation, mutation, fanaticism, corruption, and misery. Oh, and closedmindedness - doncha go forgettin' the closedmindedness." Phony Texan accent aside, there's a point to the perception - Warhammer is grim and dark, and it's also, once you get down below the magic and the scifi, very real, in a way few RPG settings attain.
No, not "realistic" - the magic and scifi kinda puts paid to that. It's "real", in that the people are very realistic - they strike true in a manner humans seldom do outside of history books and Pratchett novels. The people of Sigmar's Empire, and its sister civilization, the Imperium of Man (the sexist nomenclature in this instance perfectly reflecting the entity itself), are very hard indeed to nail down - they are stupid, brave, naïve, stubborn, enlightened, backwards, urbane, closedminded, corruptible, contemptible and worthy of respect, all of them in combination or alone. Above all, they are human, all too human, struggling against unfathomable odds and losing, some of them dying, some of them turning traitor, some of them closing their eyes and going on about their business, and some, perhaps the largest fraction, fighting back, blindly, ignorantly, without weapons or knowledge, to the bitter end, often for little gain.
It's a dark mirror of an ugly, yet glorious humanity - in the case of the Imperium, a humanity that long since left its heyday behind and is fast fading, perhaps taking the last, best hope of victory against Chaos with it, but just won't admit it, even to themselves; and, in the case of the Empire, filled with potential, right on the brink of true greatness, in the form of the Renaissance and the modern world, but all too likely never to achieve it due to the threat of Chaos. Both cultures are doomed, and though they don't know it consciously, they seem to be aware, in their hearts, that victory is a longer road than ever, and defeat a razor's edge away. And they react to it appropriately.
More than any other fantasy setting I know, humanity in Warhammer is engrossing and interesting. They have a culture, a character, and an identity, one full of foibles and with as many reprehensible traits as admirable ones, if not more, and one not afraid of voicing its opinion, whether we'd agree with it or not.
I realized how engrossing I find the human cultures of Warhammer when I came across a stack of WFRP books recently, and the first book I eagerly cracked open was not Dwarfs: Stone and Steel, but Marienburg: Sold Down the River - instead of reading about the Dwarves, my favourite stock fantasy race for quite a while, I chose to read about fantasy counterpart Holland. No other setting has ever made me eager to read about Holland, fantasy counterpart or no, so that's one plus in the margin of GW.
Anyways, enough rambling.