Glad you asked.
The answer is: A book filled with rules relating to evil characters (primarily NPCs).
The misconceptions of what the Book of Vile Darkness were so far afield that I'm surprised the book did as well as it did (it sold, and continues to sell, I'm told, very well--not that that benefits me in any way, but that's another issue...). I'm pleased that it did well, and am gratified to see it on a lot of DM's lists of favorite supplements.
But I never set out to write a book on mature roleplaying. That wasn't a part of my proposal at all. I proposed a book full of monsters, items, feats, prestige classes, yadda yadda, all geared toward evil characters, for DMs to give to NPCs. And, you'll see that's what indeed I wrote.
There were widespread misconceptions about the book:
1. That it would be some kind of guidebook for mature roleplaying.
2. That it would contain only the most horrible, evil, depraved stuff you could imagine for your game.
However, neither of these misconceptions were really the fault of the consumers.
After I proposed this book, and WotC decided to do it, people at WotC told me to make it really vile and nasty. I said that it would certainly have vile and nasty stuff in it. They said it would have a "for mature audiences" type label on it. I thought that was OK, because some material in it might be offensive to some people. Then, months later, I see the catalog copy and it gives the book this salacious spin, and implies that it will cover a wide variety of mature themes.
I shrug, and write the book I agreed to write.
I turn the book over, and get the feedback that it's not vile enough. I object, fearing the book becoming gratuitious. I'm actually told (really, this happened) to: "add in more sex and violence, but not in a gratuitious way."
So I explain what "gratuitious" means.
The book is then accepted, and kept pretty much intact. WotC added some stuff about torture methods and equipment, but that was it. However, they continue to push the book as being "mature." The book, even before it comes out, causes a huge uproar and upsets many people.
Then the book comes out, and most people calmed down.
But back to those two misconceptions. I set out to create a book on game material for evil characters. Some--but not nearly all--of the material might be offensive to some readers and players. So in that way--and that way only--I would say that the book is for mature gamers. Not because the material is "mature," but because mature people can handle potentially offensive material appropriately. There's absolutely nothing "mature" about the cancer mage, but a mature gamer knows when and when not to add such a thing to his game.
As for the second misconception, clearly (I hope) not all the material in there is as depraved as one could make it. The vast majority, I think, can be added to a D&D game and not even really change the tenor of it. Curses, poisons, death-dealing spells, and so on are part and parcel of the game already. My goal was to create interesting and useful material. Some of it is fairly nasty, but much of it is no more depraved than the assassin or blackguard classes in the DMG.
However, both misconceptions led to some people being disappointed in the book. Either because it wasn't the guide to mature gaming that they wanted, or because it wasn't depraved enough. I feel badly about those disappointments, and I wish the book had been pushed for what it was, rather than what some people apparently wanted it to be.
So, when people talk about an "article on _genuine_ mature themes" (as opposed to what they found in the BoVD, I assume), or claim to write some evil material that is "far more vile" than the BoVD, I never really know how to react, since they play off of what many thought the BoVD was about, but in my mind never was.
Thanks for indulging me.
Edit: Took out some too-harsh comments about WotC.
