Good overall improvement from the previous versions
There is a new leveling system, whereby the experience gained is higher when your pokemon's level is closer to the opponent's pokemon. It allows the player to explore more pokemon rather than sticking to only the 6 pokemon that you're holding. In mid-game, you'll be given the mystery egg which will greatly boost experience gain even further, and you'll level roughly every 3-4 hard battles.
The battle tower is replaced by a battle subway. It's just the change in name, however the system is still the same. You earn battle points after winning a consecutive 7 battles. However, now you can choose from the easy railway line or the hard railway line. At the 21st and 49th battle, you'll face the battle master of the railway and earn more battle points. This system should be improved so that there is more end game replayability and development.
End game levels for your pokemon is now higher at level 7x-8x. The 2nd time you face the elite four would be the "official" one, and their pokemon will be at 7x. The intermission level gap after the first faceoff is a huge gap from 48-50 to 7x. After the first elite four faceoff, the wild pokemon and trainers have a sudden level jump to 6x and some grinding will be needed, or you'll be struggling. It is now easier to raise your effort value points to maxed as it's easier to reach lvl 100.
The entire layout and display has changed to a more interactive landscape with seasons, and some vibrating pixels that signifies rare pokemon. The design of the buildings have become more 3-D and a more realistic environment, like moving vehicles and ships. However, it is wasted there are no major battles or events at such places, and it's usually just a long walk alongside the beauty of it.
Overall, pokemon black is worth playing with it's new battle systems (single, double, triple and rotational battles), seasonal changes, underwater ruins, dreamworld and new pokemon moves and outlook.