We have noticed that the fullscreen mode of the HTML editor causes usability problems for our users. A typical behaviour is as follows:
1. Edit Page resource
2. Switch HTML editor to fullscreen mode
3. Make desired changes
At this point the user feels they have finished, so they start looking for the 'Save changes' button, but can't find it. They then decide that they need to close the editing window (note that in Moodle 1.x, this would be the right thing to do), but rather than click the Fullscreen button again, they simply close the browser window, and lose everything.
I am unsure of the best way to improve this but it seems to be problematic as it stands.