Diablo II allowed you to, regardless of what class you are using, to add to any stat you wish to excel in. This opens up new combinations for classes. Before you assume that barbarians can cast Firewalls or teleport from one place to another, think again. Thats because of the Unique Individual Skill Trees which each class has. Barbarians, for example, have no spell casting capabilities, but have War Cries, or Weapon masteries.
Potions are also back and improved.
For example, the mana potions now have 5 types. This also applies to the Health Potions. They also have a new life restoration system. The more signifigant one being the ability to run. A problem which plauged Diablo 1 were the slow moving characters in the game.