For today there are a number of great game mechanics that I could go with. But I know just the one that I will pick for today. First I would like to give a couple of honorable mentions.

Savage Worlds- Bennies

Re-Roll: If you roll a test and don’t like the result, you can roll again and keep whichever result is better.

 

Recover from Shaken: You get a free Spirit roll to recover, but timing matters.  A Benny has the same effect as a Raise on the Spirit roll and let’s you take full action immediately.

 

Soak damage: Wounds suck!  You can spend a Benny to make a Vigor roll and reduce the number of wounds you take from a single attack

FATE- Aspects-

An aspect is a phrase that describes something unique or noteworthy about whatever it’s attached to. They’re the primary way you spend and gain fate points, and they influence the story by providing an opportunity for a character to get a bonus, complicating a character’s life, or adding to another character’s roll or passive opposition.

And you can use Fate points in game for

Invoking an Aspect: This allows you to either reroll the dice or add +2 to your roll, whichever is more helpful. 

 

Declaring a Story Detail: Allowing you to add a detail that works to your character’s advantage in a scene.

 

Compel: Sometimes (in fact, probably often), you’ll find yourself in a situation where an aspect complicates your character’s life and creates unexpected drama. When that happens, the GM will suggest a potential complication that might arise. This is called a compel.

But if you hop in the way back machine you find this horrid rule found in D&D THACO (TO HIT ARMOR CLASS ZERO).This was the absolute bane to many gamer’s existence. It was cumbersome and slow, Not to mention annoying.
So my Favorite Revolutionary Mechanic simply falls to D&D change from the THACO system to the TO HIT mechanic.  Lets take a quick look at THACO

( From the D&D WIKI)

THACO is an abbreviation for To Hit Armor Class Zero (0). To calculate if a hit succeeds you would take the AC of the target and subtract it from the attacker’s THAC0, then roll a 20-sided die; if the die rolls equal to or higher than the calculated number, the attack hits. That is, THAC0 − AC = roll needed to hit.

This means that a good character will try to get both their AC and THAC0 values as low as possible!

Example: If the player has an AC of 5 and THAC0 of 10, and the enemy has an AC of 10 and THAC0 of 18:

Chance of player hitting the enemy: 10 – 10 = 0. Although it may seem that with these numbers it is impossible to miss, a roll is still required as a roll of a natural 1 is ALWAYS considered a miss.

Chance of enemy hitting the player: 18 – 5 = 13. this is a difficult target to hit, since a roll of 13 or above is required.

Another example: If a target has an AC of −3 and the attacker has a THAC0 of 8, 8 − (−3) = 11. The attacker must roll 11 or greater to hit.

Fighter classes have the best THAC0 progression and spell users have the worst. Lower THAC0 is better, since you have to roll equal to or higher than the to-hit number.

For more on THAC0, consult the AD&D 1st/2nd edition DM Guide(s).

A rough guide to convert THAC0 and AC to 3.0 and higher is to subtract the number from 20. so 20-THAC0= Attack bonus, 20-AC= (New)AC.

using the Above examples the Player now has AC 15 and attack bonus of +10, and the enemy has AC 10 and attack bonus of +2

SO with the launch of 3rd Edition came the To hit Roll or Attack Roll. Simple and easy.

An attack roll represents your attempt to strike your opponent on your turn in a round. When you make an attack roll, you roll a d20 and add your attack bonus. (Other modifiers may also apply to this roll.) If your result equals or beats the target’s Armor Class, you hit and deal damage.

And AC was determined simply as well.

Armor Class
Your Armor Class (AC) represents how hard it is for opponents to land a solid, damaging blow on you. It’s the attack roll result that an opponent needs to achieve to hit you. Your AC is equal to the following:

10 + armor bonus + shield bonus + Dexterity modifier + size modifier