Actually, while we're talking about AH....
After playing through Act III all night on Nightmare with Viz and PD and facing the perfect challenge and just barely scrapping by all the wins in a satisfying way, I had like 100k sitting around with nothing to do so I figured, "why not check out the AH for buying and see what it's like".
So I sorted by the stats I wanted, for each piece of gear, then ordered them from min buyout price and while most people want crazy 30-100k buyouts, if you browse around you'll find great items that are much better than your own for 3k-15k a piece. I bought like 5 pieces of gear for about 40k total and my DPS went from ~2400DPS to 5000DPS.
Then I jumped back into the last level I had just played with everyone, the final quest in Act III, and yeaaaaaah with 5000DPS barbarian with HP leech, it's exactly like Normal mode again....only even easier! Enemies go down in 2-3 hits, elites in 5-10 seconds, bosses in 30 seconds.
I can see how using the AH can screw up your enjoyment because it's easy to get really overpowered and you lose out on the gradually power increase that matches the gradual difficulty increase built into the main game. It also takes away the LOOT, because for 95% certainty, nothing you find in the next 10-20 hours will beat the stuff you just bought from the AH. So you can't look forward to each new piece of loot with hopes it'll be better than what you've got.
Now on the higher difficulties like Hell and Inferno, I think it's less of an issue because you need everything you've got; especially for Inferno. But for Nightmare, AH is really overkill.
People are forgetting that in order to even use the AH properly you need to have sold enough/made enough money to buy the wares on offer. It's hardly a win button.
I bought a really kickass bracer for 2500

And even my weapons, which are best the absolute best DPS possible for my level range were only like 15k (some people wanted 200k for the same weapon). Prices aren't that high if you look enough.