Blizzard hotfixed in a significant change to Diablo 3 multiplayer over the weekend. Previously monster health and damage would stack depending on the number of players in the game, now Blizzard have completely removed that damage buff. Monsters will still have more health, but they won't be any stronger. That applies to co-op games across all difficulty tiers, and is excellent news for groups getting wiped in seconds by top tier hero mobs on Hell and Inferno.
The hotfix was announced on the Diablo 3 blog along with a few other minor fixes, including news that smashed vases and ash pots will now drop slightly less gold , HOW COULD YOU DO THIS BLIZZARD?
Here's the hotfix list. There's a mild spoiler there if you haven't played through Act 3 yet.