In Diablo 3 for the PC, PS3, and Xbox 360, just like in the old days in Diablo 2 (with the Players command) you can change the Monster Power Level (aka MP) to increase difficulty of mobs outside of the standard difficulty level, and increase the XP you receieve from them in turn (as well as boost Magic and Gold Find when playing in Inferno.)
You can use this to help powerlevel characters a lot faster that normal. Be careful however, because these monsters will be out for blood. You don’t want to set it higher than you can handle. Setting the MP level too high and you’ll end up dying more often, which won’t help you much.
The MP level can be set from the very beginning at level 1, and is set independently on each difficulty setting.
How to Change MP in Diablo 3
1. Open the Options menu and go to the Gameplay tab
2. Check the Monster Power box
3. Open the Quest Selection window
4. Choose the Monster Power level from the drop down box near the bottom under the Difficulty
5. Click the Select Quest button to confirm your settings
Remember that if you join another players game, you’ll be set to their MP level while you’re playing with them. Once you leave their game, you’ll be set back to your own MP level.