One of the biggest complaints about last year's SimCity was its forced online aspect. Even if you weren't actually playing with other people, you had to be connected to one of Electronic Arts' servers, which for weeks after launch were spectacularly unreliable. Consequentially, there was no provision to save or load historic city versions. This placed an extremely high cost on experimentation because if you screwed your city up, there was no going back.
Offline mode was widely demanded by the game's players, and modders swiftly discovered that the game itself could be made to run offline, but only after it had loaded a city from the server.

EA has finally promised to reverse its original decision and enable an offline mode. The tenth major update for the game, Update 10, will bring offline mode with full local loading and saving of games.

As a consequence, this move will make the game a lot friendlier to modders. EA has been asking modders what they'd like to be able to do with the game to customize it and extend it, but the online aspect has imposed a number of constraints. Mods can't fundamentally change the way the game works, because they could disrupt online interactions. With an offline mode, those restrictions should be largely lifted.

Unfortunately, other issues with the game—in particular, the tiny maps—are unlikely to be addressed any time soon, with EA previously claiming in October that even after "months of investigation" the performance of larger cities means that the "vast majority" of players wouldn't be able to use them