One of the most significant gameplay differences between the original versions of Final Fantasy IV and the new DS remake is the radical transformation in how the game's major bosses fight. Scarmiglione, Cagnazzo, Barbaricia, Golbez, and many less significant bosses have so far all been given brand new counterattacks in order to increase the game's difficulty. Many regular enemies have similarly been given new or enhanced counterattacks, to a degree that I have never seen in another RPG. However, the results of this experimentation are actually really good; the addition of counterattacks as a major part of most battles has radically increased the amount of strategy that I have needed in order to win compared to earlier versions of the game, and has made many fights much more dynamic and interesting than they originally were.
Counterattacks are a pretty simple way of making up the gap in the number of actions a boss has compared to a large party of characters. If a boss is limited to making only one action in the time frame where each party member is capable of taking one action, then it is at a significant disadvantage and will be quickly overwhelmed by even simple strategies. This is why many RPGs, such as Final Fantasy 3 or Dragon Quest 8, have bosses capable of making 2 or 3 actions in a single combat round. However, the Active Time Battle System makes it harder to judge how many actions either side is going to make in a certain time-frame, and making multiple actions in a row would look inconsistent under the ATB system. Many later Final Fantasy games got around this by giving bosses multiple parts, each of which could attack independently. However, it would have been too inconsistent with the original game to completely rebuild boss designs to accommodate this method. So, giving bosses lots of counterattacks was a pretty elegant way to overcome the player's number of actions and make fights harder.
The addition of counterattacks has pushed FF IV's dominant strategies in a completely different direction than the original game and even most Final Fantasy games. In most entries in the series, the dominant strategy is a scorched earth approach, where the player continuously pounds on a boss with physical attacks and the party's best magic attacks. The counterattacks in FF IV DS force the player to adopt a much more conservative and defensive strategy instead. For example, Barbaricia counterattacks with an all-character lightning spell whenever she is hit with an attack. If a player was to recklessly attack her with all of his fighters, he would find himself listening to the Game Over music in short order. Instead, it is necessary to focus on defense and healing, so that the party can safely absorb the counterattacks without dying. Other bosses who counterattack with negative status inducing spells similarly force the player to focus his energy on restoring those debilitating status conditions. The fact that FF IV DS has this different dominant strategy is actually really refreshing. It makes the game feel new and exciting instead of the typical Final Fantasy experience.
A particular benefit of having bosses (and regular enemies) use lots of counterattacks is that it makes power storing abilities useful for the first time. In FF IV, Yang has an ability called Focus where he sacrifices a turn to store up his energy so that his next physical attack will do double damage. While this ability has been in not only many Final Fantasy games but other RPGs as well, it has never been useful. Spending an extra turn to do double damage only does as much total damage as making two regular attacks in a row, and performing two regular attacks always has the added advantage of at least doing some damage on the first turn, in case the enemy only has a few hitpoints left. However, the addition of reliable counterattacks makes a single big attack advantageous over several weaker attacks; fewer attacks means fewer opportunities for counterattack. So much to my surprise, Focus went from being a move I never used to an important element of my strategy.
I also need to say that Barrier Shift, a staple of the Final Fantasy series, works much better as a counterattack move than as an actively used power. When Barrier Shift is actively used, it results in two problems: the boss wastes too much time Barrier Shifting to actually fight, and it is so unpredictable that it is likely going to frustrate the player into just ignoring it and focusing on non-elemental attacks. Making Barrier shift a counterattack solves both of those problems. Since the boss doesn't have to spend a turn Barrier Shifting, it will be focused on doing damage throughout the fight. Furthermore, the fact that the boss will only Barrier Shift as a counterattack is predictable enough for a player to actually go to the effort of scanning the boss's weaknesses and exploiting them.
As a final note, one of the best features of the counterattacks seen in Final Fantasy IV DS is that they are always explicitly marked. Every time an enemy uses a power as a counterattack, it is marked Counter: Attack, Counter: Lightning, and so forth. This way, the player knows for certain that an attack is done in reaction to one of his actions, and can plan accordingly. Without this clarity, it is possible that a player could get the mistaken impression that the boss was just doing all of its actions at random, and end up getting frustrated from constantly falling into the enemy's trap. A boss who counters every physical attack is hard enough on its own without forcing the player to figure it's pattern out blindly.
All told, the boss fights I have seen so far in Final Fantasy IV have been really fresh and exciting thanks to the newly added counterattacks. My only complaint is that it might be a little too consistent, since it is fun to beat a boss down with lots of basic attacks now and then after all. I am really curious what the next several bosses in the game will fight like.