I, for one, liked 3e BoVD. There was certainly talk of sex, but it definitely wasn't the main idea of the book (outside of the art direction). Over the course of our 3.5 gaming, we've come to use the sacrifice rules, souls as power, and a bunch of the spells. Sure, there were times when a character wanted to become a thrall of Orcus and thus needed to get the lichloved feat. But as a rule we faded to black whenever sex is involved, whether it's with the chaotic good barmaid or the chaotic evil mohrg.
With that being said, I don't want 4E to just be an update. I want it to be it's own book, and my preference is to keep it as a DM book in the likes of Demonomicon. Technically, 3e BoVD was also supposed to be for DMs only, but as player-NPC rules were the same back then, there was a natural overlap that players could take. With 4E things are more clear-cut, and as such a purely-for-DMs BoVD becomes possible. A focus on fluff regarding the DnD brand of evil (I don't need RL evil personally, because it's still a resource I'll use for a game), some new monsters, monster themes, a few delves, and a few "really evil NPCs" like the dread emperor would be nice.
I would also like to echo the sentiments to tone down the art. The main flaw of the 3e book IMO was that there were exposed boobies in just about every chapter opening art. And in most cases, they didn't even have to do it - they just wanted to shock and awe. It was sex for sex's sake.
At the same time, I don't want them to shy away from sex for the sake of shying away from it. If Fierna's the archdevil of lust, then by all means, her space in the book should be about evil sex and its implications.
In short, for the art and fluff text:
Sexy Fierna, and Belial's "roots," -> Makes sense. That's their evil shtick, after all. (Hehe, stick)
Naked sorceress with amputated mammaries -> No if it's not in context
Hag Countess cleavage -> Why, WotC, wwhhhyyyyy??? (thank the gods she's dead)
PS. A "mature" treatment of Malcanthet would have been nice, but since succubi are devils now, I doubt we'll see her in this edition.