One of the most unique game elements of the Super Robot Wars series has always been the Spirit Command system. Every character in the game has a list of six Spirit Commands, which each has its own effect and SP cost. So long as the pilot has SP, these commands can be used freely without giving up any kind of action. These effects range from minor, but incredibly useful boosts such as Accel (increases movement range by 3 for your next move) and Focus (increases hit and avoid rates by 30 for one turn), to very powerful abilities that can be extremely useful for complex strategies, such as Zeal (allows you to take an extra turn) or Attune (increases an ally's hit rate to 100% for one turn).
The importance of these abilities to the game can not be understated. Because most units can only survive somewhere from two to four hits from a normal enemy, and a lot of units can't survive a direct hit from a boss at all, using Spirit Commands such as Focus can be essential for units to survive battles and quickly defeat enemies. Also, because so many bosses have very high defenses, incredibly large hit point totals, and can regenerate as much as 30% of their health every turn, Spirit Commands that increase damage and enable more attacks can be the only way to even win at all. The last few bosses requires the whole team to use every last drop of SP in order to win by even a narrow margin. It would not be a stretch to say that SRW:OG2 is a game built around managing your use of SP.
One reason I like the Spirit Command system is that it puts a lot of control over the flow of the battle into the player's hand. Unlike other games, the player always has a choice to either take a risk by attacking with poor odds or to spend some SP and get a guaranteed success. The innate chances of success are an important part of the game, but they can be controlled. If it is particularly important or dramatically appropriate, a player can always choose to ignore even impossible odds and accomplish a task. In a way, it allows the game to be completely unfair and present a challenge which seems hopeless at first glance, but still allow the player to succeed.
I also like the way Spirit Commands help differentiate characters and clarify character roles. There are around forty Spirit Commands in total, but each character only has six. Four Spirit Commands, Alert, Strike, Focus, and Valor, are nearly universal among the characters. meaning most characters only have two or three slots devoted to the less common commands. As a result, the few unique commands given to a character define the character very well. A character without Focus will not be able to dodge a large number of successive attacks. A character with Guard (reduces all damage to 1/4 for one turn) is likely to be good at using units with high defense and a barrier. A character with abilities like Attune or Faith (restores an ally's health) is a support character who should be protected and kept alive. A character with Zeal or Love (the ultimate Spirit Command) is an important primary attacker.
Effects like these, which don't cost a turn and allow a player to control the flow of battle, can be a lot of fun, and in the case of Super Robot Wars, they help emulate a more cinematic style in which sudden strokes of luck and fate can affect a battle just as much as the skill of the characters.