This expansion to Age of Empires II: The Age of Kings is one that you can't go without. You'd be missing great content.
Look at this the way you would at the Brood War expansion of Starcraft. There's really not very much core gameplay added, just some memorable single player campaigns and additional content. But the content is very necessary and *cool*.
The gameplay is more rounded out and made more interesting with the addition of the Native American civilizations of the dark age times, along with the the conquistadors or *conquerors*, the Spanish.
If you get The Age of Kings, don't go without The Conquerors. (And check out my review of The Age of Kings)