Improves on every aspect of the first game, while adding a few new features of it's own.
Golden Sun: The Lost Age is played from the view of 4 characters whose goals oppose those of the original main characters. I won't go into detail (spoiler reasons), but sometime in the game the two groups meet up. Now if this is your first time then the original characters will have preset items and skills, but if you have beaten the first game you can actually transfer your data between the games! There are two choices, password and link cable. The latter has the obvious advantage, being able to transfer all the data in a few seconds, while the password to transfer everything (well, not your character names) is over 200 letters long! That is a slight pain.
A few new class-changing items have been introduced, but the summoning system is what has received a big change. Now, in addition to the standard mono-elemental summons, there are new combination summons that induce additional effects, such as sleep and healing. These summons must are not automatically earned, but must be found in secret spots throughout the game.
The last bit I will note is that the difficulty of the game has been upped a little. The original Golden Sun seemed a tad too easy at times. The final bosses this time prove they have the stuff, and you need more than constant leveling up to beat the last boss.
Final line: Snag both this and the other game, used copies are only $5-10 for a game that's worth a million.