Many rpg have a "collector" feature where you trade found items with an NPC and receive items in return.  The entire collector operation is almost always optional and the items you receive not required to complete the game.  Well, the "Dragon Pearl" thread in Tenmilli operates exactly like the typical collector feature, EXCEPT, unknown to you, you must gather ALL the Dragon Pearls and trade them in order to complete the bonus material.  In practice, this means that you better not miss any treasures along the way, or you will have to do a lot of backtracking.

Many rpg have a gambling feature, where you place bets and try to win gold.  Almost always the gambling is entirely optional.  There is a gambling game in Tenmilli, and it is set in the Arena town.  In most rpg with an arena, the arena battles are optional.  So it is natural to expect the Tenmilli gambling, set in an arena, to also be optional, and certainly you are given no hint that you must win the reward medals to complete the bonus material.  To make matters worse, you don't learn about this requirement until near the end of a somewhat convoluted dungeon.  You will be required to leave the dungeon, complete the gambling game, return and traverse the dungeon from the beginning, in order to clear the dungeon.  I don't mind having to complete a task or get a special item (like a key) to unlock a dungeon, but placing the "unlocking"  at the end of the dungeon and requiring starting the dungeon from the beginning is something I really don't appreciate.