The heart of Golden Sun lies in its combat system, which is easily one of the best I have ever seen. Combat in Golden Sun is built to be dynamic and flashy. Not only does the game give the player access to lots of fun powers, it encourages the player to actually use these powers, even in regular battles. Yet, the player still has to utilize a lot of strategy and think his moves through carefully.
In Golden Sun, characters have access to three main types of power: Psynergy spells, Djinn powers, and summon spells. However, each of these powers operates on different, yet interconnected systems. What makes the Golden Sun system work so well is that all of a character's abilities are renewable. For example, Djinn powers are almost free to use. When the player uses an equipped Djinn's special power, the Djinn is switched from set mode to standby mode. Between battles, the player can simply open the menu and switch the Djinn back to set mode, making them ready to be used again. So, Djinn do not use up any permanent resource (though there is an opportunity cost involved I will get into later).
Psynergy spells work like typical magic spells seen in other videogames. When a character uses Psynergy, the spell's cost is deducted from the character's total number of Psynergy points. What makes Golden Sun different than a normal RPG is that Psynergy points recover as the party walks around in dungeons and on the overworld. It seems to be balanced so that the characters can afford to use about one Psynergy spell per battle without reducing their average totals. The result is that the player can use fancy group attack, status condition, and healing spells as part of his regular strategy and still have plenty of Psynergy left over for boss fights. So the player is strongly encouraged to make use of these powers regularly, but not go overboard in any one normal battle.
Topping off the core set of special powers in Golden Sun are the summon spells, which can only be used when a set number of Djinn of one element are in standby mode. While these summons are easily the strongest attacks in the game, they do not cost Psynergy points to use. The only cost of using summon spells is in their opportunity cost. Djinn are not just useful for their special powers, they also provide passive stat gains and additional spells when they are in set mode. They can even change the class and overall abilities of the characters they are equipped to. So, when a significant number of Djinn are in standby mode, as they need to be in order to summon, the character suffers significant stat penalties. Furthermore, summon spells exhaust the used Djinn, which require time before they can recover and return to set mode. If the player uses a powerful multi-Djinn summon in a regular battle, not all of the Djinn used will recover in time for the next fight.
The dynamic quality of Golden Sun's battles arises from the interconnected nature of these three sets of powers. When a fight starts, a character has all of his or her Psynergy spells and Djinn powers available. As soon as the character uses one of the powerful Djinn abilities, the character's available options change. Now, the character is down one option as far as Djinn powers go, has reduced stats, possibly has different available Psynergy, and now has a low-level summon spell open. So over the course of a protracted fight, each character's options will likely change significantly. Unless the player limits himself to only using basic attacks or psynergy spells, every character will be using a new action every turn of battle. Thus, battles have a flow to them that keeps the action from being repetitive or stagnant.
The other outcome of this system is that it opens up many different strategies and supports many different play-styles. For example, the player could use Djinn powers in every battle, reseting the Djinn afterwards. Alternatively, a player could intentionally switch his Djinn to standby before fights, and start every battle with a powerful summon attack. One player may equip the Djinn with the intention of maximizing stats and creating the best classes and Psynergy, while another player might arrange the Djinn to set up summons quickly during boss battles. The possibilities are nearly endless.
My only complaint about Golden Sun's combat system is that it doesn't do anything to make items particularly useful compared to other RPGs. While there are smoke bombs, sleep bombs, and the like, there is little point to using them. I already have access to group status condition spells, so single use status condition items are not very valuable. In fact, since money seems to be very tight in the game, spending money on healing items and bombs seems to be a sub-par choice. Maybe this will change as the game continues on.