Don't Be Afraid of Advantage
Importing 5e's Advantage/Disadvantage mechanic into B/X helps "rulings, not rules"
This is a pretty simple and straightforward post, about a B/X houserule that made it into my system, and why I think it’s superior to (most) numeric bonus/penalty mechanics.
The System
At the bottom, this is really simple: first, throw out any situational modifiers that would add a bonus or penalty to a roll, like a +1 or a -2 or whatever.
Then, learn to categorize — on the fly — any task that the players are attempting into one of five simple categories:
Automatic: there is no point in rolling, it just works
Easy: this may or may not work, and skill and luck both matter, but chances are good that an average-skilled adventurer could pull it off.
Normal: this may or may not work, and skill and luck both matter, and this situation is about “par for the course”
Hard: this may or may not work, and skill and luck both matter, but chances are worse than normal.
Impossible: there is no point in rolling, this has no measurable chance of working.
Once you, as a referee, can intuitively sort any situation into one of these five categories, you’re pretty much done. This is the essence of “rulings not rules”.
Mechanically, automatic, normal, and impossible are already straightforwardly incorporated into the B/X rules — either you do a normal attack/save/n-in-6 check, or you don’t bother because the results are a foregone conclusion anyways.
“Easy” and “Hard”, of course, are where 5E’s advantage/disadvantage comes in. An “easy” roll is just “roll two dice and pick the better”; a “hard” roll is just “roll two dice and pick the worse”.
This has the advantage of still having the same range of possibilities as a normal roll, while skewing the likely outcome towards the top or bottom of the distribution.
It has the second advantage of being really damn easy for the players to understand.
It has the third advantage of being easy for you to understand. You don’t have to look at four or five different bonuses and ask, “does this one really apply here?” All you have to do is just imagine the situation and let your gut tell you “yeah that sounds easier than normal” or “no, no way in hell will that work, not if they try it the way they just described it.”
See? Rulings, not rules!
