In 3rd edition, it wasn't necessarily a big deal if two rooms' worth of monsters attacked you at once. The CR 12 monster in room A4 and the CR 12 monster in room A5--well, that's still just an EL 14 encounter and only incrementally more difficult than those two rooms tackled separately. But in 4th edition, it really feels like something that's twice as hard. Which isn't to say it's impossible--my Thursday guys might have pulled out a victory with a little more luck and a little more foresight. But you can't blithely kick open door after door, that's for sure.
He analyzes the numbers in this post.