I've been running a Labyrinth Lord campaign since last May (PCs are at 5th level) and have been continuously creating the gameworld as I go along. My players are not much concerned with game mechanics, and love to be included in the creative process, so they don't mind me shifting rules around on them. For instance, I just went back to original standard d6 weapon damage about a month ago and the guys just said, "okay", and started modifying their character sheets. Part of their buy-in comes from the fact that their ideas for the feel and look of our gameworld frequently become canon, and our gameworld determines the rules, not vice-versa.
So I'm interested to see whether, and to what extent, other Lab Lords have modified the rules to reflect the reality of their gameworlds. Have you changed out the missile weapon system or clerical spell system to reflect the way your gameworld works? Allowed monster races as PCs? Etc.
Just to clarify, I don't want to create my own game. I love B/X and LL, with all my gamer heart. I suppose this type of thread begs the question, "How many house-rules does it take before you make a new game?". But my intention is to make small to medium-sized alterations that don't cause incompatiblity, but rather change the emphasis here and there.