I think this is the first time that this is being mentioned (anywhere): big parts of the interface are being scripted, so there will be some quality modding on the interface (the kind of quality that can be seen in World of Warcraft mods).
The syntax being used is the one from Python.
The script will access the game through a SDK. Also, the scripted code can access the whole of .Net Framework classes (though it's not that necessary).
The editing of the scripts is done inside a special window, that allows the editing of the scripts while the game is running (edit & continue) with text auto-complete and jumping to function definitions, the inspection of the game variables in real time (through a property grid) and the complete description of all the exported classes (inheritance, properties, methods, events, inner types (enums, other classes)). Please note that not ALL game classes have been exported to the scripts.
Also, please note that not ALL the interface has been scripted, due to game (and time) limitations.
But the road has been opened for you to unleash your imagination and talent.
And, as always, there is the Mission Editor for single missions and dynamic campaigns.
At last, but not least, the way the AI thinks has been scripted and is being stored in text files (in a custom format). So that has also been opened for modding.