http://www.worldofwarcraft.com/burningcrusade/

An increase in the level cap to 70

Two new playable races, including the magical Blood Elves

New starting zones in Quel'Thalas and beyond

The entire new continent of Outland, reachable through the Dark Portal

Many new high-level dungeons to explore in Azeroth, Outland, and elsewhere

New flying mounts in Outland

Many new and dangerous monsters, including epic world bosses

Hundreds of new quests

Hundreds of new items

A new profession: Jewelcrafting

Socketed items

And much, much more...