Before we go any further, I must admit that the Diablo series was something I never really got into (this is primarily due to the fact that Diablo 1 and 2 were never available for the Macintosh, which was my family's computer back in those days), so I am rather neutral about this issue personally. However, Several friends of mine have expressed everything from "meh" to flat out anger at this move - and so I was nudged to comment on this issue.
To start, here is why Blizzard feels this is a good move:
Executive vice president of game design Rob Pardo notes that the wealth of improvements and features Diablo 3 brings to Battle.net necessitate the always-online requirement. Specific additions that he refers to include:
A persistent friends list.
Cross-game chat via the RealID system.
Persistent characters that are stored server-side (no more having to play online once every 90 days, nor item duplication cheats).
Persistent party system.
Player-versus-player and public game matchmaking.
Dynamic drop-in/out for co-op
Larger item stash that gets shared among all of your characters (at the moment, up to 10)
The auction house, outlined here.
The Achievement system and detailed stat-tracking, both of which feed into the final point:
The Banner system, a visual way to display your prowess in the game. Banners start out like emblems, where you can choose from an array of symbols, patterns, and overall shape/design. Then, you can tweak its appearance through Achievements and other accomplishments. Examples Pardo cites include whether the character is in Hardcore mode, how many Achievements have been earned, how many PVP victories, and so forth. Additionally, the Banners also have gameplay features; in-game, rather than use Town Portal, you can click on a player's Banner to instantly teleport over to said player.
But at the same time...
While Pardo recognizes that people sometimes want or need to play offline (such as internet outages, or playing on a laptop during an airplane flight), he notes that the increased security, plus benefits like the above, outweigh those other concerns. "I want to play Diablo 3 on my laptop in a plane, but, well, there are other games to play for times like that."
Really? "Hey thanks for spending $60 on our software, now go play something else".
On one hand, Blizzard makes a good case with it's list of reasons to keep the game online, but on the other - Why should a single player title *require* 24/7 internet access?