Love ‘em or hate ‘em enrage timers are in. Bashiok has posted on the matter in this thread, on the official forums:

I know what some boss enrage timers are, but to save spoilers I’ll transpose it onto the Skeleton King. So, as an imaginary example, in Inferno if you don’t kill the Skeleton King within four minutes he starts spawning three times the amount of skeletons. That creates a much tougher situation, but it’s one you could very well handle if played right.

People kiting the Skeleton King around for an hour with their level 3 characters is fun to see. It’s not fun when it’s level 60′s at end game and item drops matter. To make these kills and thus a character’s progression matter we need to try to fight cheesy tactics that allow people to just flop through the game.

UPDATE: More input from Bashiok.

The point of the game is efficiency… Killing monsters as quickly as possible to maximize your time to find the drops you want. That is the game. I think you should do whatever you think is fun, absolutely, but if you want to go at a slower pace, not really worry much about maximizing efficiency, and just kind of take it at your own speed you’re going to have a pretty rough time in Nightmare let alone Inferno. Hopefully as the game progresses and difficulty ramps up you’re naturally being encouraged to tighten up your build and scrutinize your gear more and more.

In any case though, as I detailed the enrage timers aren’t “I Win” buttons for the bosses, so a more defensive build should still be able to progress if they’re geared and being played well.

…

With the Nephalem Valor buff the best loot is off champions, rares, and (assuming the buff is up) bosses. As I said the point is ensuring “legitimate” progression. But if someone wants to farm rares and champions in the beginning of Act I because they can’t beat the Skeleton King, they should absolutely do that. And actually, everyone should be expecting to do exactly that when they reach Inferno.

…

They aren’t “I Win” buttons.

…

Nope! You nailed it, that’s the intent of an enrage timer.

I like how you throw in “artificial” as a negative. It’s a video game. It’s all designed and artificial. If there’s a “natural” or “organic” way to stymie someone’s attempt to cheese their way through a boss kill I’d be happy to hear it.

…

Not a big Diablo fan, huh?