I will defend the covers of the core books. I think they do three great things for D&D gaming:
1. They don't shout, "Geek." This means that gamers who might once have been a bit nervous to carry around books that a lot of people consider either geeky or morally unhealthy can now read books in public places. They're more marketable this way, because a book store won't have to worry about customers being offended or just overlooking it because it seems like a pulp fantasy novel. Take a look at most fantasy novels, and then look at most novels on the Best Seller lists. Few are fantasy, and few have illustrated scenes on their covers. As a 'gateway book,' the core rulebooks are a lot more marketable and respectable.
2. They're different. We're not playing 2e anymore. Though neither cover style is indicative of a particular style of play, it was very important when the game first came out to make it distinctive visually. A clear visual distinction from 2e (meaning no more art by Jeff Easley, who seems to be getting worse and worse at painting people as the years go on) helped nudge D&D into the 21st century. Sadly, they decided to use Easley on way too many covers nowadays, but that's just my artistic opinion.
3. They don't pigeonhole the game. If you buy a PHB and see it has a knight in shining armor on the cover with a rogue skulking in the background and a sorceress in a thong, you'll subtly be receiving the suggestion, "This is what D&D is." But it's not. D&D is a very diverse game, that can be played in nearly any setting. Oriental Adventures, African, Modern, Sci-fi, even bizarre realms wholly removed from our world. I'm glad the 3rd edition designers didn't use a cover image, because honestly, the subject matter is far too diverse, and any cover would do a disservice to the content.
Thus, I basically divide covers of books into 3 categories, because we do honestly judge books by their covers, and a good cover should convey the content of the book.
Adventure books should typically have a cover that presents something from the adventure, so that DMs will know at least vaguely what they're getting into. Look at The Forge of Fury, or Queen of Lies. Both have covers that let you know what you're getting into, at least in some way. Compare that to The Return to the Temple of Elemental Evil. Just some guy in a hallway. That's not really what I was expecting, and it really doesn't evoke anything for me. Whereas I have the cover of The Forge of Fury on my computer, and I use it sometimes as wallpaper. Great piece.
Sourcebooks should have covers that represent the tone of the book. If the book is of relatively limited scope, then a cover image typically works fine. The Slayers Guides, or
Tome & Blood, or Arcana: Societies of Magic all have fair covers that represent what they contain fairly well. It gives you an idea of what you'll get inside. Larger sourcebooks that are a lot more diverse in content typically benefit from a more abstract cover, like Relics & Rituals or Manual of the Planes (which still has an image). I personally thinks Traps & Treachery could've just as easily used an image for the cover, but the cover design worked fine. Then again, they could've done something like a rogue leaping through a trap in a gauntlet while the other members of the guild look on to see if he's worthy. For books like these, it really depends on the scope.
As a subcategory of sourcebooks, there are setting books. Think Ravenloft, Oriental Adventures, Living Greyhawk Gazeteer, Forgotten Realms, or DragonStar. Again, it depends on the scope. Ravenloft, I think, should've had a cover image, because it's a very unified setting. Forgotten Realms or DragonStar, on the other hand, are too huge and diverse for a single image to do them justice. When I ask for a cover for Mythic Earth (my next book), I plan to go for a cover design rather than an illustration, because no single illustration can properly represent all the myths of planet Earth. I could try to do something like a montage, like they did for the Star Wars book, but still, it'd be too diverse.
Finally, there are core rulebooks that present a rule system. You want to be able to appeal to the most people possible with these, so use the most buyer-neutral cover possible, while still having something evocative. I think the Core Rulebooks for D&D are some of the most intellectually-designed covers possible for this game. A picture is worth a thousand words, and D&D's cover thankfully doesn't say, "You have to play in medieval Europe." Nor does it exclude it. The covers invite more gamers to play, and the more friends we have in this hobby, the happier we are.
