1up recently interviewed Hitoshi Yamagami and Masaki Tawara of Nintendo's Software Planning and Development Department, who shed some light on the darker style behind the game. They decided to drop the light-hearted scenario of the game because they didn't want to depict war positively. The more realistic tone then led to different visuals and a few tweaks to the gameplay. While the fundamentals are the same, Yamagami and Tawara say that Days of Ruin will have a faster pace than before.
Because the previous Advance Wars games were great in of themselves, change to the series can seem a little daunting. Still, we like it when developers take risks, and we think the game will be all the better for it. We'll find out for sure soon enough, though, as the game is scheduled to release in a few weeks.