Below are some ideas for specific areas in the game. Feel free to add ideas as you think of them! That way even if you don't use them, someone else might. If you would like to claim an idea, please put your name on it.
A player wearing all the leaf/bark/boulder gear from Goblin Pass could gain access to a secret room, via transfer - as the goblin's think he is one of their gang. Smacking over the 'boss' in their den gives the player an item, or some treasure, or perhaps just the first step for a later quest to foil some dastardly goblin plan to do something ridiculous, like cover the Meadow in custard.
The Ultimate Dragon notices if someone is using a shield and says 'Oh, charged shield? Well, here's one I prepared earlier!' and loads its own. (See C Wear_v#_# to check a charged shield value).
Plane of Fire
Visiting the elemental plane of fire is a dangerous thing, and lords who spend any amount of time there have a chance of getting burnt by the extreme heat. Thankfully, due to their lordly studliness, this is mostly a minor annoyance to them... but over time it could add up. By wearing the Ring of the Efreet, the player is rendered immune to this particular damage prog.
Wearing various pirate gear from Mutgars while in the area has all the mobs talk like a pirate at the player...