Inferno Monster Levels No Longer Linear Throughout Inferno
This comes as a shock really, inferno is no longer equal throughout the full difficulty.

Originally Posted by Blizzard (Blue Tracker / Official Forums)

This is may be a good a time as any to REVEAL TEH SEKRITS! that Inferno monster levels aren't linear any longer. They get progressively more difficult. This was really a reaction to Inferno playtesting. Our original intent was to have a flat difficulty level where you could go wherever you want, farm for items, and it'd be no more or less difficult than any other area in Inferno. This caused a few inherent issues for us, though:
It just felt wrong. It didn't feel right to be progressing through the game and have it stay pretty much the same difficulty the whole time. It felt like a letdown to get to the final boss of the game and it be no more difficult than the first.
There’s a wide variety of players out there and we wanted to make sure everybody had something to sink their teeth into. We expect that anybody with enough time and dedication will reach level 60. But the jump in difficulty to Inferno needed to be different amounts for different people. For the crazy people they need a HUGE ramp in difficulty, for a more “casual but still hardcore” audience you want an obvious but milder increase in difficulty. So for the crazy people who play non-stop they’ll hit Act I and get a challenge, but 1 month later they’ll still have something to work on (Acts II, III and IV). For the “hardcore-casual” they will reach level 60 later and not get brick walled when they reach Inferno. They can experience some “small victories” working on Act I with the dream of maybe someday reaching the later acts.
Longevity. We know people really want goals to work towards and challenges to overcome. We made Act III and Act IV really, really brutally hard, for the most elite players only. It felt wrong to make ALL of Inferno that brutally hard.
Now, you may be saying “I thought you wanted us to be able to farm anywhere we wanted. Now we only have half as much area in the game to farm in? What gives?” Our goal is to make the loot mathematically better in the later acts without making the earlier gear completely obsolete. We feel Diablo II actually did a very good job with this and we expect Diablo III to perform similarly.


Specifically, people in D2 did Diablo runs, Mephisto runs, Pindleskin runs, Pit runs, Baal runs, etc. because the loot in Diablo is extremely random. Even though the theoretical best items might come from the later Acts, well-rolled items from earlier acts will still be better. Internally we find sometimes after an intense session of brutally hard Inferno it can be really fun to cruise through Hell Act III or IV and it’s not too uncommon surprise when an upgrade drops. We expect this to carry through to Inferno difficulty where somebody who can theoretically farm Act IV will likely still enjoy romping through Act I simply because the drop potential is still there. It’s all because of the highly random items having lots of overlap in their power distribution curves.