I've seen it mentioned that some of the Diablo 3 bosses have an enrage timer?
What exactly does that mean?
Are there any bonuses for killing one that is enraged?
An enrage timer is a mechanism in place to force you to kill a boss within a certain amount of time. Generally, it indicates that you need to exceed the minimum level of dps (damage per second) during the boss encounter.
Failure to kill the boss before the enrage timer expires causes the boss to do drastically increased damage. This means that it will very quickly destroy the player(s) if not killed within the next few seconds.
You can still kill a boss after the enrage timer starts, but you almost always have a matter of seconds to do it, rather than minutes.
Generally, there are no bonuses for killing a boss during an enrage timer (although it is possible that this is different in Diablo 3, it seems unlikely; the enrage timer is not intended as a challenge), aside from knowing that you barely squeaked by the encounter.
An enrage timer is a set amount of time given to kill a boss. In World of Warcraft, when an enrage timer was over, the boss would virtually one hit everyone to death and it was nearly impossible to kill after the timer went off. The whole idea was to make it so you had to be geared so much to defeat a boss, else it'll enrage and wipe you.
There has been just a few changes from post 1.0.4.
In Diablo 3, it buffs the boss to become even stronger. In patch 1.0.3, Blizzard chose to buff a few bosses in the Inferno difficulty. There is a ton of complaining about it. I could find LOTS of references of people complaining in official forums. Here are some of the bosses that now have enrage and what happens after the enrage timer is up:
There are no bonuses for killing any mobs or bosses during enrage. The point was to make the game more difficult instead of making it a perk like Valor.
An enrage timer is a set amount of time you have to kill a boss.
Usually after that time expires, the boss becomes very strong and will start killing people with one hit. Or it may trigger an ability that will kill everyone in the area.
As far as bonuses, it's too early to tell.
It is worth noting that this question has already been answered previously.
An enrage timer is a set time limit after which your enemy will gain additional abilities, with the intent of ensuring the fight does not continue much longer.
Since patch 1.0.4 Elite packs no longer enrage, and all boss enrage timers only take effect on inferno difficulty.
Boss Enrage Timers
Boss enrage timers appear to be specific for Inferno difficulty only
The Butcher - will set fire to all of the platforms in his chamber simultaneously if you fail to kill him within three minutes.
Ghom - will begin spawning 'Gas Clouds' more frequently after four minutes.
Zoltun Kulle - will enrage, using his 'Ceiling Collapse' ability more frequently, and for more damage after three minutes.
Belial - Enrages after three minutes in phase 3, increasing the number of green pools spawned, which will be spawned across the entire platform.
Cydaea - will enrage after four minutes, causing spiderlings to spawn more frequently.
Rakanoth - will become more aggressive after three minutes.