I've played this games for around 3 hours (non-stop). The graphical style is way more "adult", in stead of the kiddy style of the previous Advance Wars games. The story is sometimes very depressing   . There are some nice things added to this sequel like
-unit level-up, making them slightly stronger. They level up each time they defeat a unit.
-new units: Bikes , Dusters, Flares...
-CO (or maybe CO's) on the Battlefield.
-The Overpowered Tag-powers are gone. I liked the tag powers but I'm still happy with this change. It makes the game even more tactic
-Wi-Fi! Didn't had the time to try this out

So this is a f**king nice game! Must-buy (although I didn't bought it XD)