How is Emerald different from Ruby and Sapphire?

Pokemon Emerald has the following changes from Ruby and Sapphire:

  • A number of aesthetic changes are made such as the color of certain rooms or the types of Pokémon seen beside trainers
  • Each Pokémon Gym has a slightly different design except for the Mossdeep Gym which has a complete redesign
  • Team Magma and Team Aqua team up and each try to resurrect Groudon and Kyogre
  • Rayquaza is the featured legendary Poké on and plot-wise is your key to solving the main plot
  • You may select between Latias and Latios once you defeat the Elite Four.
  • There are several new islands which you may catch legendary Pokémon from the previous games. These however require tickets obtained from Nintendo Events
  • Battle Tents replace Pokémon Contests.
  • The Battle Frontier is made available.
  • Wireless linking in the Union Room (as in FireRed and LeafGreen) is available.
  • Double battles are more common
  • You may obtain the Generation II starter Pokémon from Professor Birch after completing the Hoenn Pokedex
  • Gym Leader teams have upgraded
  • A number of other features have been altered or improved.
  • A new section in the Safari Zone will be opened up once you're the Champion, this section will include Generation 2 Pokémon.