Ogrork the Mighty said:
How does Eberron keep levels down?
Actually, a combination of everything above. The most important paradigm is that player characters are a cut above the average man and are somewhat special.
There are several things that enhance this. First, most NPCs keep to NPC classes. Even the elite warriors in Sharn have mostly NPC class levels (i.e. warrior levels instead of fighter levels). The highest level NPCs in Sharn have NPC classes instead of PC class levels (18th level adept, 17th level commoner).
Second, NPCs earn experience differently that player characters. I think this actually applies to most worlds, but that depends on the world designer and DM. Basically, the 17th level commoner above earned her levels by practicing cooking for years, not by going out and fighting monste's threatening the city.
Plus, there is a conscious decision by the designers to keep this active in the world. They didn't feel the need to make the mayor of Sharn an 18th level magic-user just because they hold such an important position. The Kings aren't 17th level fighters because of their war experience.
Yes, there are high level challenges out there. For the most part, however, they aren't directly involved, yet and aren't allies or potential allies of the characters.