The WoTC covers all stink, IMO.
The BoVD is too cartoony and therefore, lame, especially for a book about EVIL.
The d20 Modern cover logo is cool, but the art is again, too cartoony and the people pictured are not attractive to look at! Ugly people don't sell anything but weight loss and plastic surgery products, LOL, because nobody wants to look at them. Yeah, I know I'll get flamed for saying so, but that doesn't make it any less true.
The City of the Spider Queen's art is technically superb--however, there are no hot drow babes on the cover and therefore I don't like to look at it (too ugly!). I want a cover like Keith Parkinson's big-busted drow babes on the rerelease of GDQ. Now that gets my juices flowing and my wallet open. Ugly girls don't sell. Sheesh, c'mon, WoTC!
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Mark--The Villain Design Handbook cover is great (I really like that artist's work), except that the villain looks dorky, with a more"Ow! I stubbed my toe!" than an "I want to murder your entire family in cold blood and rule the world" expression on his face. For godsakes, get the artist to change that guy before you publish!