The BW series introduced the new Pokémon-EX, powerful Basic Pokémon with high HP and strong attacks that award your opponent 2 prize cards for knocking them out. Originally these cards were restricted to Legendary Pokémon, but the most recent set (will be released on Nov. 6) also includes 2 non-legendary Pokémon as EXes. We know that the XY set, set to be released in January, will continue to feature non-Legendary EXes, and also Mega Evolutions. Plasma Storm also introduces Team Plasma Pokémon, along with certain cards that help just them.
From a collector's side of things, the rarer cards in BW sets are the Full Art cards (which have been present since BW1) and Shinies, which were introduced a few sets later. Full Art cards have the same text and are effectively the same as their non-FA version, but they feature artwork that takes up the entire background of the card, printed with a very nice texture. Shinies are even more rare than FAs but also have the same text as their non-shiny form. These cards have artwork featuring the shiny version of the Pokémon, as well as different textures. There were also some one-off secret rares early in the sets, a Pikachu in the original Black & White set and a Meowth in the second set.
For a more in-depth look at the competitive side of things, you can look at the
NDX-On Competitive Play forum for cards and general info (NXD = Next Destinies, the oldest set in the current Modified format), and
NDX-On Metagame Deck Discussion to get some ideas and information on the current most popular decks.