Gearing and drop rates, AH use almost required to progress.

[Athenau]

In D2 you could brute force your way through the game with pots and some patience, even with subpar gear. Farming for upgrades would make your life easier, but wasn't necessary for the most part. And getting an upgrade was much easier in D2 with the prevalence of cheap runewords.

The D3 endgame is totally different. Inferno is a hard gear check, and your equipment is all rares, which are total crapshoots (and the chance of rolling a really good rare is astronomical, just like in D2).