Rob Pardo, Blizzard's executive VP of Game Design, says that this requirement is due to the implementation of numerous new Battle.net and Auction House features.
Those features include the following:
- 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 (allows players to sell loot for real money)
- The Achievement system and detailed stat-tracking, both of which feed into the final point:
- The Banner system, a visual way to display your skills that take into account earned achievements, the number of PVP victories etc. Banners can be clicked to teleport to other players.