D&D 4 Encounters

Here's something to put your head around if you're a D&D dungeon master. 4th Edition monsters are easy to place because their rating matches that of the party. However, add two of these monsters from two encounters, and it's twice as hard, rather than perhaps 25% harder as it is in 3.x. This means that a TPK (total party kill) is much more probable when encounters combine in 4.0. Dave Noonan explains how it happened in his playtest campaign:

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.