I don't really care.
I think that sums it up, but I'll go ahead and explain a little. The Diablo games have always been about killing monsters, leveling up, and getting loot. Lots of loot. Bags and bags full. Unfortunately, a lot of that loot is completely useless to the character that acquires it due to Diablo loot largely being tailored towards certain classes. This means item trading has always been a big deal. In Diablo 2, some people eventually decided that there really wasn't anything they wanted to trade for their Uber Scything Axe of the Apocalypse and that they'd rather just have money instead. It turned out there were people playing Diablo 2 who would rather pay real money than try and find something worth trading.
This had no effect on the majority of people playing the game, as PvP only occured for people who wanted it, and most people played solo or with friends and never even saw the people spending real money for Diablo 2 items. Diablo is not a persistent world game, so you can't have the situation of item farmers camping a mob and denying others a shot at the loot. Every player (or group) gets their own instance of the world and their own shot at getting loot.
Diablo 3 is legitimizing the item trade with two auction houses -- one using in game gold and one using real cash money. There is nothing forcing you to purchase items from either of them, let alone the real-money version. Unlike an MMO, there is nothing forcing you to play with people who have geared themselves up with real money. This is pay-to-win light, VERY light. Especially since, unless they change this for Diablo 3, items have stat requirements to equip them. If you don't have the stats you can't equip it, and if you're a level 1 you can spend all the money you like but it won't let you equip that gear.
All the cash AH is going to do is allow players to do what some players would do anyway, and give Blizzard a cut. I'll be spending all my gaming time solo or with friends, so what other players choose to do is of no concern to me. If I get any good items I can't use, I might even try and sell them to recoup the cost of the game itself.
This sounds like a big deal on the surface, but only because we've conditioned ourselves to consider buying items in MMORPGs as bad, and because nobody has done anything quite like this before. But Diablo 3 isn't an MMORPG--if it was, I'd probably have a different opinion on this, but it's not, so it barely registers higher than purchasing a DLC for Dragon Age that includes new, more powerful weapons. Whatever.