One of the biggest changes in Days of Ruin that makes it differ from previous Advance Wars games is the new way that Commanding Officers are handled in battle. Previously, COs had global effects that were always in effect and CO Powers that could turn the tide of a whole battle when the CO gauge filled. CO Powers are mostly the same in Days of Ruin, but the big change is in the permanent effects and the CO gauge. Most of the changes come from how Days of Ruin introduces the idea of loading the CO onto a unit, rather than having the CO be separate from the main action.
In Days of Ruin, you must load a CO onto a unit in order to get any effect from the CO. If you don't do this, then there is no difference between having a CO and none at all (a huge change from previous games). This has no action cost for the unit (which is a good thing), but it can only be done at a unit production facility and it costs funds equal to half the cost of the unit. As such, it can't be done on a unit already in battle, and it is cheaper to make a weak unit the CO unit. Loading a CO upgrades the unit into a powerful veteran unit (about 25-35% increase in power, I think) and activating the CO Zone.
The CO Zone is the new mechanic in this game. Around the CO unit is a grey area in which the effect of the CO takes place. The size of this area depends on the CO, as well as what units are affected by the Zone and how. The in-game tutorials and menus are a bit vague on whether there is a general Attack and Defense boost to all units inside the CO Zone in addition to the CO effect, which is a problem, but the listed CO effect is very powerful, usually vastly increasing the power of certain units. Also, enemy enemy defeated by an allied unit within the CO Zone will fill the CO gauge, and as the gauge increases the radius of the Zone will increase by two squares. As such, it is very important that the CO unit is at the center of battle.
Finally, when the CO gauge is full, it is possible to use a CO Power, which costs the CO unit's action and drains the gauge completely. These abilities tend to be a bit weaker than the Super CO Powers from the last game, but they can be useful. Also, using a CO Power extends the effect of the Co Zone to all units for a single turn, which can add greatly to your army's strength. Finally, in this game some CO powers only affect units in the CO Zone, so positioning can be important.
I think this system is a great improvement over the older CO Power system, for a number of reasons. Certainly, it increases the importance of tactical positioning and movement of troops (which is one of the main focuses of the whole game), and it makes the decision about what to do with your CO and how to best defeat the enemy CO an important one. On a purely tactical level, just having a CO Zone adds a lot to the game. But beyond that, the particular implementation of the system adds a lot of choices, possibilities, and trade-offs to the game. It is cheaper to put a CO on a weak unit, and the power of the CO Zone is not affected by unit strength, so it can be a good idea to put the CO on a weak Tank or Recon unit. On the other hand, the auto-veteran status and power of the CO effect makes a strong unit even stronger, and it is important for a CO unit to live long, so making a powerful War Tank or Battleship the CO might be a good idea. Also, you have to give up the turn of the unit in order to activate a CO Power, which means that you may not get a chance to use a strong unit when you do so, which can also be a factor in this decision. This trade-off can severely add to the strategies of the game, which makes the game more fun.
Another big trade-off is the decision whether to use a CO Power or just keep the CO gauge at full, so the CO Zone remains at its enlarged size. Having a fully enlarged CO Zone is a huge benefit, and that makes it a tougher choice whether to use a CO Power or not than in the previous games, where you would not use a CO Power only if you wanted to save it for a better opportunity in a few turns (and waiting too long was usually a bad idea). With the sometimes minor benefits of the CO Powers, it means that it can be a tough decision whether to use a CO Power at all, let alone when.
One final benefit of this system is that it makes the differentiation between COs more interesting. There are some COs with weak effects but huge Zones that affect many units (such as Brenner), some COs who have moderately strong effects and large Zones but can only affect limited numbers of units (such as Will), and others who have incredibly powerful effects that only affect the CO unit itself (Tabitha). Also, the various differences in effect, and the way that attack and defense scale differently, allows there to be more variation even among Cos of similar focus. In previous Advance Wars games, there was usually people like the "Air Unit CO" (Eagle), the "Infantry Unit CO" (Sami), or the "Open Plains CO" (Jake), with one CO for each particular terrain type, unit type, or type of battle. In this game there are several COs for each particular focus, with each CO usually having multiple complimentary abilities, so that even though there are fewer COs than before there is still a lot of variety and many possibilities. The only effects that are noticeably lacking are effects which modify unit cost, mostly because it would be hard to reconcile that kind of effect with the CO Zone system, but that form of balancing may be redundant with the CO Zones anyways.
My only real complaint about the system is that the player is introduced to CO Powers quite late in the main campaign mode, after every unit and almost every major character has been introduced already. I am most of the way through the main campaign, but I have only used a CO Power about three or four times already. Unless the player plays a lot of Free Battles, he won't be able to use COs for quite a while. Besides the fact that it deprives the player of a fun game element, it results in the player having practically relearn the game and its strategies after having already mastered a lot of the system. The CO Zone has such a massive effect on strategy that it is the kind of thing that needs to be introduced to the player early.
Anyways, I am having a lot of fun with the game. Hopefully, I will complete the game soon.