While the card pool is limited, it also allows players to use cards from Magic's past that might be deemed too weak by today's standards - creatures in particular. There are SOME exceptions where cards from a cycle that were printed in a 'Modern' set are allowed, but only when these cards were printed to complete a cycle from an earlier set (most notably, the Zendikar fetch lands that completed the cycle from Onslaught).
Side note: a group of players in the Netherlands also play a format they call Ancient, which is completely different.
- Wizards of the Coast (August 11, 2008). "Casual Formats". magicthegathering.com. Wizards of the Coast.
- Decks and format information: Ancient Magic
- Information on it can be found at ancientmtg.com. There you can find some pictures, the rules, and also information as to how the format compares to other eternal formats.