You /need/ to be online?
That is utterly stupid and would be a very strong reason for me not to get into this game. Which I'm not since I'm anticipating GTA V but that's a different story...
I'm not even talking about servers which can get overloaded, but what will happen if Blizzard decides to pull the plug out of their servers in, say, 3 - 6 years because they're focusing on the game which is current then ?
Then you're talking about /not/ being able to play a /single player/ campaign because of some half brained copy protection. That is just stupid and an utter fail in my book.
And before anyone comes up with the obvious comment: "But who would want to play a game which is so old?". Well, you'd be surprised! Only last week did I decide on checking up on Far Cry 2, a game which is now roughly 3.5 years old. To my total surprise there was still some decent online gaming action going on.
Sure; not everything. For example; I couldn't get enough people together to play a good game of "Uprising" (you need to take checkpoints to dominate the map) but there was plenty of deathmatch going on. 3.5 years after release.