The online requirement doesn't affect the nature of gameplay. The singleplayer component is no different in D3 than it is in D2.
Sure it does, if my ISP or the Blizz servers go down, or Blizz Servers have performance issues, I can't play or my playing experience sucks. That's a HUGE difference.
D1 and D2 BOTH had dedicated OFFLINE Single Player modes (as well LAN and B.NET). The ONLY
reason why D3 is online only is because of this stupid AH that Blizz will make money on.