Diablo 3 is a singleplayer/co op game with a cash shop.
That requires a always on connection to prevent item duping.
I don't know if I'd say it "requires" it.
I had plenty of fun with Diablo 2 without ever buying/selling/trading items with random strangers through any auction house. My friends and I would fire up a game, play through it on the hardest difficulties and get items we needed without ever having to connect to Battlenet. When we got bored, we'd make alternate characters to try other skills. D3 has taken away much of what made D2 fun for us. Not only that, they took away a major part of the Blizzard gaming history of mods. I mean, the whole genre of Tower Defense games were rooted in mods to Warcraft.
D3 is clearly focused on the RMAH. There's no denying it. The crafting items are "tiered" so they can be sold off on the AH to
getentice people to use it. They'll push in patches to hinder players abilities to farm items making the prices go up. They'll call it "balance" and they'll start to make it harder and harder to get certain stats that make the game fun in order to drive the price up on items marked with "of the blah" that will be required to complete/compete at the higher levels.
Feel free to bookmark this for later reference.