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

[Jaffa Tamarin]

If you're patient you can probably get through hell with self found stuff, though there'll be long dry spells between levels where a new craft opens up.

You can't really progress through Inferno with self found gear though.

I finished hell difficulty with my witchdoctor using only self-found items, single-pass full-clear played entirely solo. It seemed more challenging than an equivalent run in Diablo 2 -- I died a lot more than even my most variant Diablo 2 characters. Also more fun -- more entertaining elite/champion fights (I like the increased variety of elite abilities in D3 over D2), and less time spent clearing areas where nothing posed a challenge.

And then I stepped into inferno, and yes, it is clearly an entirely different game. Fortunately I don't feel I need to play inferno in order to enjoy the normal-nightmare-hell game (which some people seem to have a problem with).