AC8 brought esc-to-cancel, a standard functionality in most software. In AC9, esc is way more useful.
Esc will activate the arrow tool. Unless...
There's a marquee present. If so, esc removes the marquee. Unless...
Elements are selected. If so, esc deselects. Unless...
There's a command in process. If so, the command is canceled.
In other words, with a command in process, a marquee present, and a non-arrow tool active:
The first escape cancels the command. The second deselects. The third removes the marquee. The fourth activates the arrow tool.
Former autocad users will want to redevelop their "esc-esc to cancel-deselect" habit.
Further strikings of esc in future releases of Archicad will close the project, quit Archicad, shut down the machine, and drive you home. Maybe.