I am just about to get this game. (I already have Sapphire.) So I was wondering..

Several changes to Ruby and Sapphire have taken place in Emerald. Below are just a few important ones:

1. Team Aqua takes precedence in the beginning of the game, pulling off the petty stunts such as stealing the Devon Goods.
2. The Pok�Nav feature Trainer's Eyes is replaced by Match Call, which allows you to communicate with people that you register. Any trainers that would normally be registered in Trainer's Eyes will automatically register after battle, but those that are not added although you battled them because you defeated them before obtaining the Pok�Nav must be approached again as in Gold, Silver, and Crystal. (Personally, I think Trainer's Eyes is better, but I see that Nintendo tried to apply a little more from the previous generation of games than just the Battle Tower.)
3. Speaking of the Battle Tower, the Verdanturf, Slateport, and Fallarbor Town contest halls are replaced by Battle Tents, which take on a battle mode similar to that of the Battle Tower. For example, the one in Slateport is Battle Swap, an elimination-style tournament that allows you to exchange your rental Pok�mon at the end of a successful battle.
4. Both fossils can be obtained, although the one that you did not initially pick (before the Pok�mon League) will appear at the top of a tower in the Desert following defeat of the Pok�mon League.
5. The Contests have been moved to Lilycove City. Progress is schematically the same.
6. The house in Mossdeep with the locked door has been replaced with a terminal for minigames that can be played with the wireless adapter with another Emerald version or Fire Red or Leaf Green. The old house's new location is Sootopolis City.
7. Kyogre and Groudon are now acquired after the Pok�mon League. Kyogre is in a cave underwater to the east of Lilycove, and Groudon makes its home around Mount Chimney. Both are at level 70. (Rayquaza is captured normally, although it makes cameo appearances during the course of the game.)
8. The style of trees and most houses has been changed, as have the dialog boxes displaying the location.
9. The dialog box font has been changed.
10. The Battle Frontier is now accessible after defeat of the Pok�mon League. This area hosts several battles whose themes differ from venue to venue, challenging core areas such as skill, bravery, and luck.

Nothing much except you can catch different Pokemon like Groudon instead of Kyogre and you get to fight team Magma instead of team Aqua

You get to fight the gym leaders twice after you beat the Elite Four and champion which is now Wallace who used to be the eight gym leader, after you beat the league you can catch Pokemon frm kanto and johto, theres the battle frontier, you can catch all legendaries, bigger Safari zone and there's some more I forgot

This game is different because in this game it seems like the gym battles and everything are the same which is kind of true but you get to battle them over and over after you beat the Pokemon league and they call you to challenge them again. Also you get to catch Groudon and Kyogre, you get to choose if you want latios or latias, and there is a new thing called the battle frontier.