What will I miss if I skip Pokemon Black/White and go onto Pokemon Black 2/White 2?

There are enough differences between the two to warrant playing both versions, but they're few enough that you can jump straight to White/Black 2 without missing out on anything major.

The region, Unova, is generally the same as the first games, however some of the differences are:

• New locations are accessible
• Your journey starts in a new town called Aspertia City (Hiougi Town in Japanese).
• The Unova Pokedex features 300 Pokemon from the start.
• You will be able to capture pokemons from previous generations from the beginning.
• New forms of the three roaming Pokemon from Black and White: Tornadus, Thundurus and Landorus.
• A new story that takes place 2 years after the events in Black & White
• All of the gyms are either modified or new
• The Pokewood movie-making minigame is a new feature to go along with Pokemon Musicals.
• Previous Gym Leaders and Champions from other regions can be battled in the World Tournament.
• Keldeo obtains an alternate form by learning the move Sacred Sword.

Possible Spoilers:

Source