Action Combat

Action Combat is a gameplay feature in Diablo III, aimed to make the combat more dynamic and give players a reason to battle at faster pace, using their surroundings.

Originally, it was exclusive to the Reaper of Souls console version of the game, but as of patch 2.4, it was added to PC and Mac as well. It is available to all players, regardless of whether they have Reaper of Souls or not.

In-game
Action Combat gives players various bonuses for meeting special criteria in battle. All of them are displayed graphically mid-screen.
 * Killing Streaks (multiple kills within short time, 15+ foes) increase experience gained from these kills once the streak is over. The more enemies killed at once, the larger this bonus will be. All experience from kills made during the streak will be accounted. Example: killing 30 enemies that each award 100 XP will grant double experience, to a total of 2 x (30 x 100) = 6,000, while killing them normally would award 3,000. The timer refreshes every time any damage is done to an enemy, displayed as a burning fuse.
 * Object Destruction Streaks (destroying breakable objects) will grant a 30% increased movement speed bonus (not multiplicative). The more objects are destroyed at once, the longer this bonus will last.
 * Environment Kill Streaks (killing monsters with traps and destructibles) will grant double primary resource gain, multiplicative to other similar effects. The more enemies are killed by environment, the longer this bonus will last.

Kill Streak (Massacre)
Scoring a Massacre will provide an experience bonus, as stated above. The more you kill, the higher your bonus will be. Each tier of Massacre has a title, which will show in its cue card when you hit the limit, and after the Massacre ends. Here are some of the names and limits needed to get a multiplier:
 * 10 kills, Massacre: 1.5x
 * 20 kills, Vengeful Massacre: 1.75x
 * 30 kills, Dominant Massacre: 2x
 * 40 kills, Crushing Massacre: 2.25x
 * 50 kills, Brutal Massacre: 2.5x
 * 75 kills, Savage Massacre: 2.75x
 * 100 kills, Ruthless Massacre: 3x
 * 150 kills, Vicious Massacre: 3.25x
 * 200 kills, Bloodthirsty Massacre: 3.5x
 * 400 kills, Diabolical Massacre: 4x

Destruction ("Crushed")
Destruction bonus starts at 7 objects destroyed. As soon as the 7th object breaks, you're awarded the "Dimensional Speed" Buff, which will start ticking down, unless you've "crushed" more objects. "Crushing" more objects will add onto the timer, but only to a limit. For a 7 streak, another one with 10 seconds remaining will only add the missing time. Sometimes it's hard to know how many you've "crushed" due to Massacre's callout taking priority. Timer caps out at 1 minute, but you can keep it there longer with more "crushes" in a row.

Environment Kills (Traps)
Trapping a foe means to use the environment as a weapon. Usually a wall propped up by some wood or a wire rack full of hot coals, but explosive barrels and any other traps will work too. As stated above, you gain double resource charge called "Trap Mastery". The time rates seems to depend on the type of monster killed, varying from 30s to almost 1 minute.