Today's art is all about the action--and then some! Cram as much emphatic intensity into a painting, goes the current reasoning. It's a reflection of the age--music is mastered to leave no "open spaces", to be as loud as possible regardless of what the music sounds like. There is little room to breathe; it is an assault on the senses. It goes beyond mere action, which was amply illustrated in old school art (see the classic "Bridge of Sorrows" painting for an example of an action shot that still leaves room to breathe).
I'm going to take a whack at placing 4e's art in my discussion of the shifting aesthetics of D&D eventually, but I haven't had enough time with it yet to feel like I can speak definitively on the topic. Like David, I'm not wowed by it, but it also doesn't me want to poke my eyes out with a stick. (Well, ok, the Sorrowsworn comes close.)
I think I'm willing to hesitantly posit that there's a strong Games Workshop influence in the art. I'm getting a strong, "see how cool" vibe from many character portraits and combat scenes that just rings similar to Warhammer art. But I'm also reminded of something Odyssey wrote recently, inspired by reading the 1e DMG:
As I was writing about the assumption of a long term campaign in 1st edition, it occurred to me (at least partly) why that is. The game is about the world, not the characters. That's why Gygax keeps going on about "the milieu," that's why the game is supposed to last as long as possible. Because the point is the world that everyone is building.
I'm not sure it's quite as emphatic a character-vs.-setting dichotomy as Odyssey's writing implies, but there's certainly some truth to that statement. And it's revealed in the art. Whether it's the "you are there" style of Elmore or the fever dreams of Otus, yeah, those works are about creating a world. You get a sense of place and culture, a hint of a reality that may be alien to our own, but still feels solid and possible.
4e's art has a much stronger focus on the actions of individual characters. Lots of characters exist without any or very minimal backgrounds. I think I'm even seeing a difference from WAR's wall-o'-action pieces in that the heroes of 4e really pop more, and command the attention and the foreground. These characters don't need backgrounds or settings unless they serve to accentuate the action and individuality of the characters. You don't get many detailed backgrounds because detailed backgrounds are not what the art is attempting to communicate. The art is speaking about the joys of playing a cool character in D&D, of being the focus of the story and the linchpin of events.
To my eyes, this leaves the characters feeling ungrounded, floating without restraint in the eye, untethered by culture or environment or any sorts of preconceptions that could limit their potential. That makes them harder to accept as "real" but does make it far easier for the viewer to adopt them as their own. Much like the simplified facial features of anime characters, a lack of detail invites viewer empathy by avoiding details that might be off-putting.
Anyway, those are just my random thoughts to-date, but I don't own any 4e books and so I really haven't been able to immerse myself in the art of the new edition to the point where I feel confident about anything I've written about it. But yes, eventually, I plan to add 4e to the discussion of the changing aesthetics of D&D.