I've got all the games from Diamond on and really enjoyed Black, but it seems too similar. What are the differences?
You can find them on Bulbapedia. Here are the most important:
- New locations such as Aspertia City, Virbank City, Floccesy Town, Lentimas Town and Humilau City have been added to the game.
- New characters are introduced.
- All Unova Gyms receive redesigns, including newly remixed music.
- There have been several changes made to existing locations, such as Driftveil City and Nimbasa City.
- Several attacks have received new battle animations.
- The battle menu and the C-Gear have been redesigned.
- The Unova Pokédex has been expanded tremendously to accommodate older Pokémon.
- In place of Cold Storage, there is now the Pokémon World Tournament.
- The player no longer starts in Nuvema Town, instead starting in Aspertia City.
- Chili, Cress, Cilan, Lenora and Brycen are replaced by Cheren, Roxie and Marlon as Gym Leaders.
- Bianca gives the player a Starter Pokémon.
- A new Team Plasma has formed, and is run by Ghetsis.
- Iris is the new Champion of Unova.
- Ghetsis returns and plans to freeze Unova with Kyurem.
- Neither the version mascots Black Kyurem or White Kyurem can be caught within the story, but can be obtained post-game by following certain requirements.
- Another new attraction introduced is the Pokéstar Studios, located in Virbank City, where the player takes part in many different movies.
- Several areas that were previously limited to the post-game of Black and White are now part of the main story. In contrast, some of the areas that were formerly part of the main story are now limited to the post-game.
- A new feature called the Memory Link is introduced, which reveals flashbacks from the NPCs that shed light on events that happened between Black and White and Black 2 and White 2.
- Many TMs and all but one HM are received in different locations.
- The Move Tutors are back, and as with Pokémon Platinum, they teach most of the moves that were in that game for Shards, but here they all ask for shards of a specific color. They also teach many moves that lost their TM status.
- A number of legendaries from Hoenn and Sinnoh (excluding the Weather trio and Creation trio) can be found in the post-game.
- Zorua and Zoroark, which were previously accessible through Nintendo events only, can now be found within the game without any special requirements.
- Hidden Grottoes are introduced and scattered all over Unova, giving access to many items and Pokémon with their Hidden Abilities, many of which were previously unobtainable.