I've been playing around a bit with DCUO, and... I really don't like the art style.
Most of the colors have the same level of saturation, which means sometimes it's hard to see what's going on. Also, half the characters look like globs of plastic rather than people (Try making a female character and put her in the "flirty" pose like half the players in the game... impossible, back-breaking poses ftw!), and the in-game faces are kinda ugly. And the apparent lack of face editing options means everyone looks damn near the same (though since most comic book artists are guilty of "Only Six Faces" syndrome, perhaps it's appropriate)
By contrast, I think the character design in Star Trek Online is much better, and that's basically what CoH's style was, just less detailed.