You know... I really dislike that picture that's being toted around the topic, the one with the adventurers fighting the red dragon. I think it would make a horrible cover for the 4E PHB.
The art is archaic. Now, I'm not dissing old-school style. But that picture is a prime example of the limitations of older artwork. It's possible to create pieces with an old-school feel but with a refinement that particular image lacks, and suffers for. On top of the generally outdated feel of the art itself, the dragon is simply horrible. Human-like arms, no structural support for the wings... It's simply not a believable or enjoyable image of a dragon. If I saw a book with that cover sitting on a store shelf, I would pretty much immediately dismiss it as something from 20-30 years ago, and likely not even give it a second glance.
And, with that part done... I actually like the new cover, and like it more than the original one. Guess I should explain why.
The original piece (the one with the Tiefling) feels so... static. Like the characters are made of wax. They have no life to them, and this is apparent in no more obvious place than their faces. The Tiefling is completely expressionless, despite obviously being in (or getting ready for) a fight. His partner isn't much better off... while her face is twisted into an aggressive, angry snarl, her eyes feel lifeless and unfocused, and distract from the experience. The ambient lighting also distracts from the feel of the piece--they're underground, but the place is lit up brighter than Time Square.
Now, onto the final piece. I like it. I really do. The wizard has the slightly wide-eyed look of someone quickly trying to figure out the best thing to do, while the Dragonborn's eyes are narrowed and his mouth just slightly open, as if eagerly anticipating the perfect moment for him to lunge forward and bring down that massive sword of his. Their poses are also much more appropriate for a fight than the poses in the Tiefling picture--the wizard has her staff held in front of her to help fend off incoming blows, while the dragonborn is presenting his off-hand, heavily armored shoulder. The scene is bright, but the colors are more subdued, and the fog obscuring the background gives the scene a better atmosphere (IMO, anyway). The presence of other characters in the background gives it a more "traditional" D&D feel, as it's obviously just not the two of them alone, and it shows something important is happening for them to have moved that far apart... likely a battle on two fronts, which I draw the feeling off from the fact that the other characters aren't looking towards their comrades, but off to the right of the scene, far behind them. All in all, it has a much more action-oriented feel, which I think is just right for a game like D&D.