As you begin to go through the larger dungeons you will sometimes notice a scrolling text announcement like those that tell you about price changes in the shops. Instead of a price change, this will be an effect that applies to the current dungeon floor you are on. Some effects are useful, while others are very bad for you indeed.

Known effects at this time:

Adventurer attack power doubled!
Enemy attack power doubled! Affects traps as well.
Attack power doubled for everyone! Affects traps as well.
Adventurer defense power doubled!
Enemy defense power doubled!
Defense power doubled for everyone!
Adventurer movement speed increased!
Enemy movement speed increased!
Movement speed increased for everyone!
EXP gain has doubled!
SP consumption halved! Appears to round down, so 5sp skills cost 2.
Consumables now twice as effective! Does not apply to Ferromin - no free lunch!
Consumables now half as effective!
Adventurers now nearly blind! The adventurer's visible area is reduced to a small oval in the center of the screen. This can be somewhat compensated with the minimap.
The map of this level has been revealed!
The automapper will not work here. Enjoy!
The floor is now slick! Permanent - you can't wait it out like the trap.
This level is full of traps. Enjoy! Appears to not affect chests, only floor traps.
Will-o'-Wisps will appear soon. Enjoy! They not only appear faster, but also in greater numbers.