WARNING-SLIGHT SPOILERS AHEAD!
The Advance Wars series has always been a beacon of excellent design and storytelling on the GBA and DS. Turn-based strategy is a niche market even on the PC, and on consoles there are very few that are well known, unless you count things like Final Fantasy Tactics. So it's to the credit of Nintendo that they've managed to get this series right. They combine an excellent story and character driven single-player experience with a truly addictive and limitless multiplayer.
Dark Conflict brings a lot of new stuff to the table. Firstly, the world and characters from the last few games are completely gone, and the game itself has gone post-apocolyptic. I loved this, but they I love anything post-apoc. In the ashes of a world devestated by a cataclysmic meteor shower and dust cloud, a few survivors live on, scavenging for food and continuing old hatred. Ed is your first Commanding Officer, and is the only survivor of the Laurentian Military academy that he studied at, and is then rescued by the First Independant Laurentian Batallion under the command of Captain O'Brien and his subordinate Corporal Lin. I rate the story as one of the best in the series, partly due to it's post-apocolyptic nature, but also partly because it's less obvious who the bad guys are. In all three of the others, the answer was always 'Black Hole did it.' The characters also get better development through various cutscenes, and it's the first time in the series that I remember them killing major COs off.
The game, like all the others, is heavily mission-based, and you can gain points for completing each mission based upon your speed, power and technique (basically: how many turns it took, how many things you destroyed, how many of your things were destroyed). I was a little upset to see that the system that you could use in Duel Strike to spend points on upgrades for your COs was taken out, but it means that the focus of the game shifts to the units themselves as opposed to the people commanding them, making for a slightly different style of gameplay. On that note, the power differences between units has decreased slightly. In Duel strike, a Mega Tank was incredibly powerful, and could take repeated batterings from bombers and live. Infantry were next to useless against it. Now, the differences are still there, but less pronounced. Infantry have a role to play besides captures, which is good. Indeed, you can make a coherant strategy out of an infantry wave. This is something they tried in previous games with COs like Sami and Sensei, but never managed because of the focus on the CO themselves. I still bemoan the lack of infantry with decent anti-air capabilities, but there are enough anti-air units to go around.
The role of COs in this game has been dramatically scaled back. CO powers are far rarer and are no longer as game-ending as they could be in Duel Strike and the GBA games. The individual abilities of COs are also less pronounced. Of note is the ability of COs to command particular units. For example, you can plut your CO in a tank, and they will generate a command zone. In this zone the attack and defence of all unit is somewhat increased, but make sure to protect that unit. Leave it exposed and you might just lose. This is a nice touch, but doesn't come into play all that often.
The unit roster hasn't changed dramatically. What were fighters are now interceptors, and the unit called fighters are a sort of hybrid of battle copters and the old fighters. There are missle-boats which basically act as landers with a single attack, meaning that transporting of troops across large bodies of water is significantly easier. There are motorbike units, which are basically fast-moving infantry, and various units have small tweaks in how much damage they can do and how much punishment they can take.
Basically, if you're a fan of the Advance Wars series, this is an addition that I heartily recommend. It has all of the creative storytelling and tactical gameplay that has made the series great so far.