Those that do buy the expansion will get two new playable races and new content for five additional levels from 81-85, but all World of Warcraft players will get to experience tons of new content in the old zones.
Deathwing's return has had an immeasurable impact throughout the Eastern Kingdoms and Kalimdor. Players will notice drastically altered terrain, thousands of new quests from levels 1-60, and updated level ranges for some zones to improve the questing flow.
Additionally, new race and class combinations are available, though the Golbin and Worgen classes will not be live until the retail release of Cataclysm. Also, those of you still not at level 80 will get a nice bonus while you wait for the expansion: "experience required to gain levels 71 through 80 has been reduced by 20%, which increases leveling rate by 25%."
For more patch notes including details on specific classes and more, head on over to Blizzard's snazzy new World of Warcraft community site.
It's a smart move to release so much new content for everyone and not just people who buy Cataclym. I'm sure lots of old-timers who quit for whatever reason will come back and play.
...whatever reason = to live in the real world :D