At the moment I haven't the Dragon expansion, but the other big expansions give you the possibility to set game difficulty:

• EASY: take the talisman and defeat the Eagle king (Higland expansion)
• NORMAL : take the talisman and reach the crown of command (no expansion needed)
• HARD : take the talisman and defeat the Dungeon Lord (Dungeon expansion)

Without the Talisman you can't win.

What do you think?

Comparing only to Talisman itself and not other games, I'd say this is loosely the way we see it. In general, though, and without consideration of a talisman, the Dungeon isn't that hard and easy to reach the CoC than through the base game. Hence we don't play with expansion boards anymore unless:

1. We have the extra time for the extra setup, and, conversely,
2. We want a shorter game time that just the main board unto itself.

Sometimes easy also includes consideration of game time to a win. Both expansion boards tend to shorten the game for us.