I think this is an important point that Obryn is making; my issues with 3.5's CR system was that it was often comparing apples to oranges in terms of difficulty. There's already a bunch of random chance in the game thanks to using a d20 to accomplish most any action; don't compound the problem by allowing that "luck" to determine whether a player lives or dies (thanks to, for example, a lucky critical hit by a level 1 orc for 2d4+4 x 2 damage).