Sparkleberry,
Using the Option key produces 3 different behaviors in Excel, Word, and PowerPoint. See fourth paragraph below:
The continuing issue is a conflict bug between Yosemite (OS X 10.10) and Microsoft Office for Mac 2011 (Version 14). With the latest versions of Office (14.4.7) and Yosemite (10.10.1), the problems continue to persist.
SUMMARY
Clicking the Green Stoplight Button in the upper left corner to go to Full Screen Mode takes you to a Full Screen Mode that is not reversible with another click of the Green Stoplight Button, because the Green Stoplight Button completely disappears in this Full Screen Mode.
SOLUTION
The solution is to use the Keyboard equivalent of the Green Stoplight Button: Command-Control-F. This behavior is true for Excel, Word, and PowerPoint.
OPTION KEY
According to Apple, under Yosemite, you can revert to Mavericks (OS X 10.9) behavior of the Green Stoplight Button by holding Alt/Option before clicking the Green Stoplight Button. However this action results in three, distinctly different behaviors in Excel, Word, and PowerPoint:
1) In Excel, this is a null command; absolutely nothing happens.
2) In Word, the widow enlarges to the full width of the zoom specified in the Zoom Window of the Quick Access Toolbar, with the stoplight buttons still atop the window; on the second Alt/Option click of the Green Stoplight Button, the window returns to its original size.
3) In PowerPoint, the window enlarges to full screen, with the stoplight buttons still atop the window, behaving as PowerPoint did in Mavericks.