One design aspect of games like Apocalypse World, Warhammer 3rd and FU: the Free, Universal Roleplaying Game that I find amazingly cool are the non-binary results. Besides with the traditional success/failure (or critical success/success/failure/critical failure), you now have the “Yes, but…” and the “No, but…” results.
Basically, a “Yes, but…” result is a success with a cost. For example, if you’re attacking the ogre, you manage to hit it, but your sword is now stuck. If attempting to persuade an NPC, he agrees to help you but only if you do a favor to him before.
A “No, but…” is still a failure, but one that gives you some kind of compensation. For example, you spear charge fails but the target loses his balance (he can either become flat-footed for 1 round or be forced to spend a move action to prepare his weapon). In a diplomatic meeting, you fail to convince the king to help your cause, but one of his hot-headed generals will help you clandestinely (good luck leaving the palace with the troops without be seen). “No, but…” results open new directions/goals. You still fail, but you get new opportunities to win instead of just failing.
As you can see, these “half-results” are lot more interesting than just a simple success or failure, especially from a narrative point of view, because they force the players to make choices at every turn (and the Gamemasters to think out of the box). They also present a solution to break the boring routine of “I hit”, “I miss”, “I hit” etc during combats.
Actually, as many other narrative tools, they aren’t strictly necessary. In fact, many Gamemasters already use these techniques ad hoc, by “eye-balling” the dice results. If a target succeeds a difficult 15 check by getting a 15 or 16, maybe that is a “Yes, but…” result. If he fails by rolling a 14 that could be a “No, but…” result. The Gamemaster’s style, the adventure’s circumstances and the campaign’s tone usually will dictate if these things will come or not. Some Gamemaster use “No, but…” for checks were it just doesn’t make sense for the PC to completely fail.
Now, a cool aspect of some modern RPGs is that they place the choice of accepting these “half-results” at the hand of the players.
An actual play example (from my friend Kazê, a Pathfinder GM): When one of his players fails a check but he believes that success is still an option (or its just more interesting), he offers that player a “Yes, but…” result. If the player accepts, his check is a success, but he must drawn from the Critical Fumble Deck and suffer its effects. The choice itself lends tension and fun to the game.
If you want this kind of complication there’re lots of ways of implementing it.
You could define that a positive margin of success of 1-3 is a “Yes, but…” result, while a negative margin is a “No, but…” effect. These margins could be bigger (-5 to +5) or could be somehow affected by a PC’s Charisma modifier. Or maybe the PCs could have a number of daily Luck (positive Charisma) and Complication (negative Charisma) points. Spending a Luck Point allows you to turn a failure into a “No, but…”, while Complication Points would be used by the Gamemaster to trigger “Yes, but…” results.
You can easily change the above “currency” to Hero or Action Points. You could allow players to spend a Hero/Action point to change failures to “No, but…” and offer such points in exchange for “Yes, but…” results (almost like the game economy of FATE).
Or just let things run random. For example, roll a 1d4 with each check. A “4” indicates a “half-result”. Or use Fudge Dices, Warhammer Dices etc.
You could even use these “half-results” as a sort of modifier. If things seen messy or very confused, don’t inflict a check penalty (the universal -2 modifier), but declare a margin of complication (like a critical threat range). Any success or failure by that margin generates a “Yes, but…” or “No, but…” result.
Finally, you can go further and establish that certain classes like Rogues (maybe through Talents) and Bards (alternate class features) can activate/ignore “half-results” a few times per session (or daily). It can also be used to represent skill mastery. Maybe such character simply can’t fail at a check (unless impossible) and the worst he gets is a “No, but…” (it’s a more interesting option than giving PCs bigger mathematical bonuses).