You won't be forced to transfer your existing Diablo II characters over to the expansion. The many new items in the Diablo II expansion will also help boost the effectiveness of the original character classes. However, on a more general level, the game introduced two powerful new types of items for use with socketed equipment, as well as other items that will further enhance your characters' abilities. The socketed items in Diablo II were interesting, but ultimately weren't very effective. You could plug magical jewels into socketed weapons and armor pieces to grant them certain magical effects.
Runes are similar to gems but give different types of bonuses, like skill enhancements.
On top of that, you'll be able to find sets of runes, which can have a dramatic effect on an item if grouped properly. In tandem with these powerful, new enhancing items, this should really help ensure that socketed equipment is both unique and powerful. Still more types of items have been introduced. Other items will pack a limited number of charges of a certain skill, meaning even those classes that normally can't use that skill may gain a limited ability to do so. You'll even be able to find powerful ethereal items.