Hey, since there seems to be at least 1 warhammer fan here, can anyone tell me the difference between Warhammer Fantasy Roleplay and Dark Heresy?
[Yeah, getting interested in Warhammer the rpg. Looked up a little bit on Nurgle and found some stuff on mutations, which just looked pretty kickass, and I would like to check out the system, but not sure which is better or even what the difference is...]
There are two Warhammer settings:
One is a fantasy setting based a little on Central Europe (and one of the major players, the Empire, is basically a fantasy German). This is Warhammer Fantasy Roleplay.
The setting is grim & gritty. Cultits that follow the chaos gods can be found everywhere (or so the witch hunters keep saying - and they are often wrong, but also often right

). Beastmen and Orcs are a constant threat. Magic is dangerous and seen with skepticism, yet there are large magical universities (and you better are a member of one, if you don't want to be set on fire by raging peasants - and even if you are, you're not always safe from it.)
Other "friendly" races are Elves, Dwarves and Halflings
The other is a science fiction setting, Warhammer 40K. It plays around the year 40.000. Humans have expanded in space, and build the Imperium. But it has become decadent and devolved - humans have highly advanced technology, but they no longer know how to recreate it. Research is basically forbidden, and techpriests only strive to keep existing technology working and recover ancient artifacts from older times. This decay begun when the last Emperor was nearly killed and is now kept alive in a kind of stasis. The Emperor is now worshipped like a god.
The Space Marines are one of the major parts of the Imperial Defenses, they are genetically modified warriors (and cloned a lot), but they were also responsible for the original attack on the Emperor - Some marines defected from him and worshiped the Chaos gods. They still exist as Chaos Marines and are a major force.
The Imperium also has the Imperial Army, consisting of "regular" recruits.
There are other races, like the Eldar (advanced, but dying race), the Necrons (a machine race that wants to destroy all life, also worshipping some chaos gods), the Orks (a plant-based lifeform that has its ability to create weapons build into its genetic code), and the Tau (a technologically advanced empire working for the "Greater Good", uniting several races beside the Tau) and more.
A powerful force are creatures from "the Warp" - Demons and CHaos gods, worshipped by the "evil" forces like Chaos Marines, Dark Eldar and Necrons.
Both settings seem to have a "law vs chaos" bend, and good and evil are not clearly defined. While worshippers of Chaos are probably mostly evil, that doesn't mean that members of the Empire are always good. Space Marines for example are known to "set examples" by killing off entire populations because of a small group of heretics hiding among them, and the Commissars of the Imperial Army "set examples" among their soldiers by killing anyone not following orders or acting cowardly. Witch Hunters in Warhammer Fantasy often hit the innocent.
Both settings are obviously pretty dark.
Dark Heresy focuses on Warhammer 40K characters working for an Inquisitor, investigating rumors of heresy or criminal activities.
Warhammer Fantasy Roleplay focuses on typical adventuring party made from people of different careers - Ratcatchers work along side Roadwardens, Soldiers and Wizard Apprentices (or Hedge Wizards), and advanc
Personally, even with Dark Heresy, I find that Warhammer 40K is not so well suited for regular RPG adventuring.
Warhammer Fantasy Roleplay definitely is. Combat is dangerous, and characters will spend a lot of fortune points to avoid getting maimed, crippled or simply killed. And there are no "minions" in Warhammer - superior numbers are deadly. But the whole setting allows for intrigue, investigation, dungeon crawling and everything else you might like in your D&D games, but everything a little grittier.
The Warhammer Fantasy Roleplay system is not overly complicated and can be understood fast, yet provides enough options. The advancement by careers is very interesting. But I haven't run or played in any long-term Warhammer campaigns, though we had one that lasted several session. If you sometimes want to break out of your favorite D&D edition to see something fresh but not too remote, Warhammer is a good choice. And the adventures we plaed were pretty good.