Greetings!
Warhammer Fantasy Roleplay!

Well, mmadsen, as you well know, I am an old veteran of Warhammer, and ran campaigns in it for many years. First, in my view, the Great Things:
Atmosphere:
Warhammer has a grim and brutal world. The cities are corrupt, and there are so many races that are cool and have very distinct personalities. Dark Elves, Beastmen, Orcs, Dwarves, Elves, Skaven, Lizardmen, Ogres, the list goes on and on. Each was either very cool to play as a character or as an opponent. Dragons just stomped you into mush, period. There are Greater Demons with demon armor and demonic hell-swords that can pretty much make any party into mush for breakfast. The atmosphere provides the tools in abundance for grim campaigns in the forbissing wilderness, just as much as there are tools for ongoing and challenging urban campaigns. The grim atmosphere provides a demanding background in which to strive to be heroic, and hopefully, survive.
Combat:
Combat is fast, furious, simple, and elegant. High level characters have no such certainty in any combat. Death is possible for anyone, no matter how powerful. The critical system is very cool.
The Nature of Evil:
Chaos is absolutely devouring in debauchery and wickedness! The awesome might of the Chaos Champions is almost stifling! And, no matter how many Chaos Champions you kill, there are always more! Necromancers, Demonologists, all have atmosphere, flavour, and awesome powers. BOVD is a pale anemic child compared to Volume I: Slaves To Darkness; and Volume II: The Lost and the Damned. Both of these books unleash the death, the sexual debauchery, the madness, the terror, that is Chaos. Evil creatures and beings are evil and utterly wicked. They rape, they enslave, they sacrifice, they murder, they drive the weak insane! They delight in torture and dragging the vestiges of humanity into the gaping maw of madness and evil! Villains in Warhammer are wicked, and utterly ruthless. They don't play fair, and they seek to conquer and enslave at every opportunity!
Rules Tweaks and Campaign Assumptions:
Warhammer upended many of the assumptions that were the problems with D&D. Weapons did lethal damage, there was no resurrection, in many ways, elves and dwarves were simply superior to humans, though despite this, humans remained a strong and viable race to play. Character careers and character development made it possible to design and play realistic characters--people that you could believe in, people that had flaws and virtues as well. The alignment system was essentially thrown out the door, and it allowed you to really get into playing a character as an individual, rather than trying to artificially live up to some nonsense ideal. Goodness and virtues were still applauded and possible, but there wasn't some kind of fiat that penalized you or benefitted you for being such, other than the personal philosophical satisfaction of doing so. Thus, richer, and more developed characters could be made that responded to the environment in more consistent, logical ways. The character careers also facilitated this by allowing otherwise "mundane" professions to become adventurers, and they did so in the context of a profession, a place in society, and did so in a way that made developing the character's personality possible and more vivid. For example, your character wasn't merely a "Fighter" but your character was a Labourer, Mercenary, Mercenary Sergeant, Mercenary Captain, Freelance, and Witch Hunter. There was a character and story build-in as to what made up your "character" and your character's abilities. The careers also meshed within the larger context of the society--there was real in-game benefits for being the listed careers, as opposed to just being a generic "Fighter" and so on. Very subtle, and very cool.
The campaigns could be played with drama and great fun with little or no magic at all. There were campaigns that went along for the longest time where no one had even one magic item, or at best, only one magic weapon each. That's it. For those who get bogged down with the "Christmas-Tree" dynamic, Warhammer represented a refreshing change of pace. One could engage in all kinds of adventures and desperate missions, and the focus of the adventures was the "mission" and who the characters were as people, and what they brought to the table as far as personality, history, skills, and experiences, rather than what roster of awesome spells they had, or the backpack stuffed with whatever magic items you can think of. Magic items and spells alike definitely took a distant back-seat to the overall campaign play and storytelling.
These things, to my mind, are Warhammer's salient strengths and virtues.
Warhammer Problems:
Well, non-magic is good for a long time, but after awhile, it too gets old, and even if you want toi change into something more magical, the magical system both for spells and magic items is pretty spartan, and rudimentary. There isn't really a lot of room for development. Thta frustration can become a larger problem over time.
Similarly to the above, the career development, over a long campaign, brings the characters to the point of all looking alike. After they all have 8 or 10 careers, they start to really look alike, and they behave identically mechanically speaking. They all have stats in the 60's, 70's or more, and they all have the same skills for example. That gets frustrating.
The campaign world, while initially a strength, and very colorful, is also a weakness. As my friend Dragonblade explains it, there are "Dynamic" worlds, and "Static" worlds. Warhammer is a "Static" world, in that it is absolutely structured, and there is little room for deviation in continents, world structure, politics, technology, magic, and so on, because if one tinkers with these too much--then you have fundamentally changed the very atmosphere that makes the campaign "Warhammer" and it ceases to be "Warhammer" anymore. It is a delicate kind of thing, but I suppose it can only really be seen after one has played int he Warhammer campaign for some length of time before such underlying principles become crystal clear and evident. These principles are strong, and fixed. They are not always immediately seen, but with time, they emerge. It thus becomes increasingly more difficult and more frustrating to do anything "other" than the usual warhammer campaign of fighting against Chaos Hordes in the Old World. The world can't really grow or change, because real changes would shift the campaign assumptions too severely, and then you would be getting into areas that Warhammer just doesn't handle well, and the very things that make Warhammer distinct and interesting--its strengths, as it were--would erode, and it wouldn't be fun anymore. In some ways, a number of Warhammer's salient strengths also carry in them the seeds of it's flaws and its ultimate disatisfaction over long-term play.
It is an inspiring game system and world, but ultimately, the designers didn't really design it with long-term playability in mind. They designed it something of a lark, as more of a beer & pretzels game, something that isn't intrinsically designed for long-term play. These are essential points that the authors have made over the years in interviews, and this lack of an ultimate, well-founded, long-term playability shows up in play over time. For short-term campaigns, it is without a doubt a beautiful game. I have many fond memories of it, but alas, the flaws show through, and it must in the end, usually, I think, be left behind for a more thorough system, or only played in a limited, short-term manner.
Such inspiration though!
Semper Fidelis,
SHARK