I previously did a long post on this one at Reaction Rolls - My Favorite Sub-System
To quote the first paragraph:
One of my favorite B/X sub-systems, and likely the one that I use the most, is the reaction roll as detailed on page B21 for retainers and B24 for monsters. A quick 2d6 and you can determine all sorts of wonderful things. Just about any type of interaction can be determined with this mechanic.
I know many people don't like using randomness to determine the outcomes of roleplaying situations. I have no problem with it.