I'm a member of the "Not Buying Diablo 3 and Actively Discouraging Everyone Else From Buying It" club.
If Diablo 3 is successful (which, much to my displeasure, it probably will be), it will show other publishers/developers that an always online system is perfectly acceptable, when it isn't.
There are still large areas where people don't have access to good, reliable, and affordable internet. It's not a matter of simply "getting better wiring in your house," it's that, in many lower population areas, broadband companies will not have service because it would cost them more money to get service to those areas than they could make off of them. So these people are left with either extremely slow dial-up, or expensive and unreliable satellite internet.
Even if I live in an area where I do get perfectly fine internet, there are still the possibility my internet going out for a period of time for whatever reason. Why should I not be allowed to play the game that paid money for just because the internet company won't get a repair person to my house for a few days?
And then there's the problem of not being able to play a game if the game's servers are down. A DDOS attack, server relocation, or a number of other things that are not on my end could prevent me from playing a game that I paid for.
And finally, you cannot pause in Diablo 3's singleplayer. Any developer that disregards this essential gaming feature deserves to be thrown down into a special layer of hell.