my typical Diablo 3 session has changed. It usually goes as follows
2. Stare at my toon and think “Man, he basically looks like every other Monk i’ve seen”
3. Check Auction House
5. Start a game or join a public game if I’m feeling saucy
6. Kill 5-10 Elites
7. Get 7 Level 55 Rares and a bunch of trash blues.
8. Don’t even bother with a boss
9. Die, because I’m bored and not paying attention to Arcane lasers
10. Look and my repair bill, and then just exit the game.
This. 1000X this. Except half the time I just go from step 4 right to step 10.5 (exit) just because I can't motivate myself to play the game. I'd rather go and work. Now, I like my work, so that's not as bad as I make it sound. But look at it from my point of view: the entire point of Diablo 3 is to get better gear with which to farm better gear. This is cyclical: you farm so you can farm better.
The problem is, with the RMAH, you can buy a "lottery win" item (1/100,000,000 drops) for $250. I can make $250 much faster by doing what I do for money than I can get 100,000,000 drops and/or farm up enough gold to buy a similar item on the gold auction house. And since all I'd be buying the item for is to farm item/gold better, why would I ever do that? I will never be able to farm items or gold as quickly as I can "farm" an equivalent amount of real money... so why play the game at all?
The RMAH has made doing my job the most efficient way to progress in the game, and it's done this so efficiently that I have no need to play the game at all, because no matter how much more powerful I become in the game.
I don't want to jump on the "RMAH ruined Diablo 3" bandwagon... but it kind of did.