The only people I've heard about getting to and past Act 3 are making spreadsheets with complex algorithms, spending millions upon millions at the AH, and still entirely based on RNG.
Math is hard.
On a serious note: Melee is EXTREMELY gear orientated. Just like Diablo 2. Just like Diablo 1. Just like almost every single RPG in existence.
From reading the forums, it would seem like the Wizzies and DH's that are farming Act 4 are mostly doing it vs Purple named mobs with 0 stacks of Neph, so they're flooding the AH with mediocre blues. In addition, Blizz's stats say that only 1.9% of the population is on Inferno. SO, the good items it would appear are from the few people in Inferno, with 5 stacks of neph, finding rares so at the moment, good inferno items are actually kind of rare. That, plus people are f-ing moronic thinking I'll pay 5 mil for a 900 dps weapon with no other stats.
Also from what I've been hearing is that DH's and Wizzies are nearing the top end of the damage they can put out. There isn't much more room for improvement. However with monks and Barbs, since there is better gear still to be dropped, their improvement will be massive. Last week on here, monks hit a brick wall in Act 2. Now I see a few are able to farm Act 3 because they got better gear.