It wasn't easy to decide from what to start. There are so many new features and they are so important and interesting! However, there is one feature that every AX 2012 developer will use for sure.
In AX 2012 the new development environment was introduced (click on the picture to enlarge).
Now IDE is separated from the application. That means more space because of missing navigation pane, no interfering content pane that can be removed only using some tricks and the development-specific main menu.
You can also notice the beautiful visual studio-like code editor and compiler output window.
Development tools are now much more accessible. Version control parameters, for example, do not require navigating 3-levels deep menu.
Development workspace can be started by typing
ax32 - development
from command line, or by
combination from the application or another development workspace.