Let's say you play HC, take it slow. Make it to 60 and through Hell. You farm Hell for gear for awhile to make sure you're ready for Inferno.
You work through Inferno in a 4 player game, get to SK (just picking a random boss), and during the fight the other 3 people die (pick a reason why.. !@#$ players.. didn't gear up.. lag spike.. whatever).
You're now stuck trying to solo a uber hp/dmg buffed boss in a 4 player game? You can't TP or leave game like in D2.. this is pretty much a guaranteed death sentence because 3 other people sucked.
Hopefully mob HP/dmg is scaled instantly as opposed to when the area loads like it was in D2? Maybe as soon as a person dies in HC the hp/dmg scale they were supplying is removed whether they leave the game or not?
Just a thought.