While I have yet to fully test this rule out in more than one quest and hoping it wasn't already mentioned previously, I'd figured I'll post it here to share.
In my games Halflings and Thieves are limited to certain equiment and are typically weaker in body points, like the Wizard. This makes them less likely to be choosen for quests. This rule gives them an edge my players like.
Movement Modification
"Because of a Halflings size and the Thief's cunning, they are the only heros that may pass over a single monster as long as there is an unoccupied space in front or behind the monster. You may not move to this space if it requires you to move diagonally."
Enjoy!