Version 2 0 » History » Version 4
One of the key objectives in the 2.x series is "unified networking" (#9), which means that the engine is always running in client/server mode, even locally in single player mode. This has major implications for almost every subsystem, or in other words most subsystems will need improvements and/or redesign before unified networking can be properly implemented.
A major reason for changing the major version number is that a separate launcher is no longer required for using Doomsday (#1600). This implies built-in package management (#1601) and all the benefits it provides for resource management (e.g., multiplayer, savegames, editing).